Traders are shopkeeper mobiles that aren't tied to a specific room. They can be set to wander, or you can have more than one of them in a specific room.

Traders are created much like shops, except that you set the Store flag on the trader, and the list of stock, 'shop.type', and every other property usually set on the store, on the trader instead.

If there are multiple shopkeepers in a location, you have to use 'from' with the trade commands to say which you mean.