They sell a variety of items. Main cities for example carry common items such as: Pots, Jugs, Shears, Buckets, Bowls, Cake Tins, Tinderboxes, Chisels, Hammers, etc. Then there are the items players sell to the store in bulk, such as Unstrung Bows.
When you buy something from the General store it works in a similar way to selling it. Instead you might buy 10 Tinderboxes and then the next ten would cost more. This continues until it gets to a fixed price.
Stores that are not labeled as General most of the time will not allow you to sell anything to it unless it is already in the available stock.