To own a shop, you must have:

level 10 construction

100 Wood

items

You can either own a shop by buying one, or can make your own. In your shop, you'll need items for sale.

EX:

At "ye ol' barrel," ale costs 360 gp.

You don't HAVE to own a shop, you can just use the Trade Center where you can sell stuff or buy stuff that people might have.