[Sky]

Try /warp buysell
/warp prism
/warp packmart

Those are a few off the top of my head, they can usually get you started.
You can set up your own shop really easily if you use the sign shop plug in, make a chest, fill with what you want to sell, and make a sign with the information that the plug in tells you to do (give it a quick google, you'll find some results) and then players can click on said sign and buy or sell items to you. You can set the price on the sign too.