What platform are you on? For iOS, Android (including, e.g. Amazon's Android-based tablets with their own storefront), and Windows Store you're pretty much beholden to accept all payment through the vendor's store (I think Android and Windows Store allow you to use 3-rd party payment processors, but generally you want to at least support first-party, because most everyone using the platform is already integrated into that payment system.) All of these platforms support the idea of consumable goods -- so you just create an item for sale that's $1 for 100 consumable coins, or whatever. If the only use of virtual currency is to buy other items from your own in-game store, then that's basically all their is to it, you just program your game to issue those goods in exchange for the coins, and the coins are used up in the process. You'll have to track who owns those items and whether they've been traded on your own. If you want to support trading of the virtual currency between players, then you probably have to do some work on your own too, since that's more complicated than a simple consumable good.

If you are not on such a platform, but are in a position to maintain servers for the life of your product, and use that as an intermediary for transactions, then its a straight-forward problem, albeit with a lot of details:

You accept real-world currency through a payment processor of your choice (e.g. paypal), the user is purchasing a good like any other, except that its virtual.

This transaction generates a receipt of some kind that says what the item purchased was, and the user ID of the person who purchased it, and a 'nonce' to prevent replay attacks, which can be sent to your server.

The server sees the receipt and issues the goods (in this case, virtual currency) to the player's account.

Once in the account, your servers manage spending and trades.

As a central 'mint' for your currency, you can simply create each coin with a unique serial number / hash to prevent double-spending. Something based on bitcoin could be an interesting avenue for a more distributed system, but that's a whole different scenario.

I came here just to praise Frob and Ravyne answers. Fantastic answers.

Ok, thanks. But maybe it is possible to realize it by own efforts? Did you ever faced similar task? How much time and money we will waste?

It depends entirely on your project, there's no way of knowing that. You'll need to design what kind of "internal currency" your game will have, and implement an external real-cash payment system by your own (Well, you can always ask for advice, but the more specific, the better).

Little elaboration - input of the real cash is just first part. Player must have ability to buy items in game-SHOPs and from another player, also players should be able to transfer money among each other. So we want to build advanced Virtual Economy system)

There's no "already made" solution to that. And, it's still not clear what you want.

It seems that you want to implement a "Trade System" where you can buy items from NPCs and from other players, but the internal currency that Frob mentioned is not specified. The players shall buy items from the NPCs and other players with in-game currency/money or real-life money or both? Whatever the case is, you'll have to implement a bridge from Paypal and the game's internal currency depending on your implementation of the game (No one better than you to say how much time you're going to take to deal with this).

Were you also wanting people to be able to withdraw real world money back from the system?

Depending where you do business there are some pretty strict legal requirements involved in this, and depending how your game operates this might fall into the category of gambling where you require specific permits and have to follow certain procedures.

Obviously, keeping everything secure and ensuring people can't cheat the system would also be a very high priority.

I don't think they handle in-game shops, but if your idea could be classified as gambling perhaps you could take a look at Betable, who offer a service that handles a lot of the legal requirements on your behalf.

Either way, if we're understanding what you're wanting to build correctly you're going to have to do a lot of custom development work for the in-game systems -- I'm not aware of any "drop in" system you can just configure for your game that does this sort of stuff.

Hope that helps!