If they made gold BoP, we'd be forced to use the barter system. It'd be rough. They wouldn't be 'Gold-Sellers' anymore, but then they'd probably just trade linen cloth for epic gems or something.

That's the problem with allowing trades between players. If barter can be done efficiently, it becomes a defacto money. If it can't be done efficiently, then there isn't a lot of point to it.

Picture a game where you only have an inefficient barter system. If there is an auction house, you could put an item up for offer, and your only option is to accept other goods in trade. Which is fine if you want goods, but not if you don't - you would then have to barter away the pig you got for the chickens you do want. And all because nobody would offer you chickens for your Sword of a Thousand Truths, because the people wanting Swords didn't have chickens.

There's a reason games adopt currencies.

I'm pretty sure a strict barter system would not work either. The gold sellers would figure out a way to make money on it. If I remember correctly, Diablo II had gold, but it was virtually worthless. The "gold" sellers therefore sold Stones of Jordan, or mats to make powerful items (don't quote me on this, its been a long time). The point is, there will always be gear or things people want. If we can trade with people to get or make the things we want, the gold sellers will find a way to provide it to the people that want it.

Blizzard has a game that runs on a barter system. This game is Diablo II: Lord of Destruction. What happened is players developed their own currencies ranging from hacked items (duped runes) to out of game currency you can buy with real money (forum gold, f~ jsp but that's another story). There are legit currencies too. Some players trade in gold to gamble, but above that middle runes or perfect gems were swapped as a currency. Players kept accounts full of characters storing valuable items and traded on web forums swapping shields for perfect gems for runes for more shields. Were Dupe methods in Diablo II fixed, and sites like jsp cracked down on, we'd still see players using a sort of currency like perfect gems or runes just as they do when ladder begins and the game world is wiped clean. Players develop their own currency if none is built-in.