Stand inside a land claim you have permission to build in.
Grab at least one of the item you intend to sell.
Crouch and right click on either a chest, barrel, or enderchest.
Enter in chat how much you would like to sell one of that item for.
As many as you can want!
Keep in mind it costs $100 (in-game money)! This is done to deter people from making a needless amount of shops.
While the plugin we use works with physical items, your shops don't need to be limited to that! Get creative with your shop and what you sell. Maybe you have a water elevator building service, or maybe you sell custom made haikus. The possibilities are endless!
By default, shops are set to sell shops. Meaning you are selling the item listed. But you can change them to a BUY shop by looking at the shop and entering /qs buy.