You might also try Armageddon, it has several merchant houses which you can be recruited into, and a pretty high amount of players for a RPI. ( for more details)

Try Batmud,a fairly intresting one,with good amount of players and a deep skills system,allowing close customisation to your views(merchant is one of the numerous guilds).

I would suggest trying Elysium. Mechant isn't actually a class, in fact it is class-less, which gives you a wider oppertunity to become anything you want. But there are many guilds, for examply the artisans allows you to rune weapons and armour, ensorcelling allows you to enchant rings, amulets and weapons and forging allows you to create items. You can sell your weapons in player run shops, or travel by ship to other continents to sell to others. You make a lot of money and it is very fun.

elysium-rpg.com

You can reach it at elysium-rpg.com port: 7777