A tax to get into the City where the best and wealthiest merchants are as opposed to the Towns where its free to enter but the selection is smaller and worse.

Just remembered, back in Ultima Online on your crafting character if you joined the different NPC "Guilds" you got a discount on items through that guild, (mage guild gave mage discounts) but you could only be a member of one guild at a time. So it was kinda like a membership..

Paying to enter a shop without providing anything extra seems to me just an annoyance to beginners with naturally low money.

The one point where it could make sense to me is if there is new randomized items in a shop every time you enter. Then it basically makes people pay for abusing the randomizer to get a better item, though I would not let the payment grow per use as people clicking the randomizer button 1000x would have to pay 1000x the amount of gold already and better items get less and less likely. Higher quality shops could have a higher price per click, so everyone can at least go into some lower level shop without being completely cut off.

That could trick people into paying ingame for their doings instead of abusing safe/load or back and forth travelling between 2 shops, if it goes faster.

I kinda see a parallel to an annoying IRL parking experience I had recently.

Paying $15 for parking is nothing in the scheme of things. Considering that I need to be in that building to tend to important business and advancing my life long plans, it is irrelevant. Yet, I realize there are some things I need that I don't have with me. And as the minutes tick by I realize I would like to have them more and more. Another $15 isn't too big a deal, is it? Though there was the parking fee from just yesterday as well. And that time earlier this week. It's all going to credit and haven't paid that bill just yet. That really starts to add up. Do I really need that stuff I forgot or can I make do with what's at hand?

IRL, my income isn't going to increase dramatically and I would consistently be thinking that sort of thing. Do I really need to be here? Have I planned for everything? What are the risks involved. Kinda sucks in life but it can have potential to be fun to think about playing a game. But in a game it's generally expected that income increases significantly across relatively short playtime. That set fee to be where you want to be becomes more and more irrelevant. Should it become irrelevant? If it does that fee is only a nuisance as it's a couple more seconds and button clicks preventing you from doing what you've made up your mind on. Nothing is going to stop you. There's no pause for reflection anymore.

So, similar to what others have already suggested, I would say that if you add something like a fee to enter a building, you have to make sure that planning what you're about to do is fun and that it stays fun across the entire course of the game.

I can think of a few ways this could work: 1. These shops have some extra complementary service - like a healing fountain, repairing facility, or haircutter. Or a tray of 'free samples' like healing potions or food items. The cost of entrance goes towards accesing these extra features in addition to the shop itself. 2. Black market shops - If you are trying to sell a bunch of stolen goods (or just things you can't normally sell) you might have to pay a fee "Ta prove you ain't just wastin' ma time.". The money you would gain from selling the items offsets the entrance fee. 3. Similarly, if shops normally have a limit on how much money they have on hand and you're trying to unload junk, certain wealthy shop owners dedicated to buying stuff might require an entrance fee but in exchange they have a much higher amount of gold on hand so it should be possible to unload all your vendor trash on them. 4. In addition to the above, they could have an extra special deal on certain staple items like healing potions, ammo, or whatnot. Lets say your average shop buys items at 50% list price and sells at 200%. Special shops buy certain items at 75% and sell at 150%. However, if you pay the entrance fee, and sell enough vendor trash that the shop owner runs out of money then he changes tactics. "Wow, you've got alot of stuff to sell. Tell ya what, I'll give you a special deal". At that point, he buys and sells health potions (and other staple items) at the same price ( effectivly treating those items as another type of money) he might also open up a list of special items or powerful/expensive equipment. Since the only way to open up these special deals is to empty the shop owners funds, the player is guaranteed to have a decent amount of money so once they achieve this they can either buy up a massive number of healing potions/items or get the rare piece of equipment. 5. Paying the fee upgrades the shop - Probably doesn't make sense in an economic sense but maybe every time you pay the fee, the shop upgrades a bit, adding features or better selection or funds. Perhaps this would be better as an investment system where every time you buy stock in the shop you can make a suggestion like "get more potions", "more money to buy items with", "install an alchemy set", or "better prices for animal furs". This would better suit it if these are one or two shops the player visits on a regular basis so upgrading them is beneficial. 6. Otherwise, I suppose if it's a one-time investment (like buying a membership), it could be a way to unlock new areas in a way. Like, the shops control the respawn areas, teleportation gates, save areas or the like and paying the fee unlocks them. 7. There is a way to eliminate the entry fee. Either paying a larger membership fee, fullfilling some condition (like sell/buy 1000 gold worth of stuff), or defeat the boss monster of the area. This encourages the player to complete this objective in the area. Such as, if defeating the boss eliminates the fee, that shows how your actions are earning you fame. 8. Fluff to show how desperate the situation is, or how important the NPC is. Say you enter a camp or village with injured people and the town healer is busy helping people. You can ask to buy stuff, but she demands a fee to open her "shop" and makes it clear the money will be going to getting more medicine. She's running a hospital, not a market stall. Similarly, while the royal wizard can teach you spells, you have to pay money upfront to show you're serious and to compensate for distracting him from his regular duties. (in both cases, fulfilling some objective may remove the fee out of respect for your actions). 9. The "shop" in question isn't meant to be a place you regularly go to. More like a club or special event. Put all the strippers, movies, booze, or mini-games in this location for when players want to take a rest from the main adventure and goof off. Maybe each town has a "inn" and a "tavern". The inn is meant for visitors and just has the beds, bar, and quest board that wandering adventures use. The tavern is meant for the locals and has better booze, conversations, and games or whatever. If you're new to town you have to pay the fee to get in (it keeps the riff-raff out). If your fame in the area becomes great enough, or you buy a house in that town, then you can enter without having to pay the fee. In-universe, the entry fee is the locals way of keeping undesirables out of the "safe zones" without necessarily barring the player from that area. Could even be paired with a karma meter of sorts. If your character looks suspicious, the guy guarding the door can increase the fee while if you look more respectable it can be decreased or waived accordingly.

A 'sellers market' where the goods are in such high demand (and the supply so low) that even the chance to buy is an advantage.

To be in such demand (like for survival) is an abormal situation where there can be only a few handled customers possible (supply runs out fast) or there is some exclusivity where lookeeloo customers are a waste of the merchants time (and people who want to look but not buy might be 'milked' as a side business).

Product with entertainment value ? Then purchase may not be the main business .

Generally there is sufficient competition (or l;ack of real need) that the merchant EXPENDS money to get prospective customers.