Based on how I played Recettear: An Item Shop's Tale: I'd sell anything and everything I could acquire for cheap or free, and milk my customers for all they're worth. Of course, I played through the whole main game (that final deadline was tough to reach!) before looking up its hidden mechanics. Apparently, if you sell to everyone cheap, it'll build trust with the customer, so later on you can jack up the prices much higher. That tidbit would have been useful, hehe. Or maybe I'm remembering this mechanic wrong, don't quote me.
If I had to specialize... I'd sell signs that you could draw or right on then post somewhere (like MineCraft or Animal Crossing). They'd sell like hotcakes because people love to express themselves and stand out from the crowd. I'd also enjoy seeing my product all over the place, and it'd be interesting to see what others came up with. So, obviously I'd be a merchant in an MMORPG.