Merchants are all about "merchandise". They love the thrill of the sale, finding an excellent bargain, or getting every last copper from an unsuspecting shopper. No one can haggle prices like the Merchant! They also specialize in identifying unknown objects since they've seen so many.
Another interesting skill of the Merchant is the ability to conceil valuables to prevent anyone from pilfering their goods (within reason of course).
Finally, Merchants have the ability to barter and do straight exchanges of items for others. This works wonders when dealing with expensive items where the storekeeper doesn't have enough money to do a deal.