There is another problem besides items being not divisible.
If for example someone plays a blacksmith and needs coal/iron and sells swords, but the miners searching/finding iron/coal dont need swords but mules for transportation, but people selling mules need to feed those and farmers selling food need fertilizer and people selling fertilizer need bags and people making bags need leather and people finding leather from monsters need a sword? And there can easily be many other such cyles, which could easily be fulfilled by everyone trading for money, but get near impossible to solve with only bartering between 2 people allowed, when everyone would shy away from accepting something they dont immediately need and they dont have experience in how good it would resell and no connections to other people needing it.
I guess people would then try to resolve it by slowly finding some easily convertible and divisible item like gems needed for upgrading, but that can have sideeffects like people hoarding those like they would with money and people needing it for its intended purpose suffer from it being less available or only when they by luck have something a hoarder badly needs at that moment.
People would also get tired of very slowly selling something and think they'll just keep it for later(in reality hoard it forever), reducing overall availability which in turn makes it easier for other people to have a near monopoly and only sell at ridiculously high prices, which leads to even less trading, which can lead to inflation.
If you want to help with trading in such a system you would need to make it very easy for lazy people. Repeatedly putting together different sets of items you want to get rid of every few hours/days can get really annoying.
The easiest solution for people would probably be to just have two sets of items in their storage account, things to keep and things to sell.
Then when someone needs something he types the name and how many of it into a search, if there is any account which contains that inside the 'for sale' set he can move a few things from his 'for sale' set into an offer box, everyone else which 'for sale' set contains it gets a message next time they are online, the first that clicks accept gets the trade. Then maybe allow counteroffers with showing the 2 boxes with offered and wanted item and 2 boxes with all other items in these 2 peoples 'for sale' sets and allowing to drag and drop stuff around and click counteroffer which in turn notifies the first person who can accept or decline.
The problem is then people would maybe feel spammed by all those offerings and reofferings while they maybe just want to kill some monsters at that time.