/shop Plugins are server-sided shops, meaning that all items are bought and sold from and to the the server instead of a player.
ShopGUI+ is probably the most well known /shop plugin out there. Kitted out with basically every feature you might need (other than a dynamically evolving economy), it is a pretty solid choice for most servers. Too bad that it's 18 USD and performs poorly when scaled to higher player counts. Only worth buying if you want support for other plugins.
Solid /shop plugin with a lot of good features. I highly recommend that you find a fork to download instead of using the old SpigotMC version.
Free, simple /shop plugin that just does the job. It also has a premium version for 12 USD. Free version is extremely limiting.
Now hear me out.. I know that DeluxeMenus isn't exactly a server-shop plugin, but it is nearly perfect for the job since it is EXTREMELY flexible. cj89898 has made a wonderful DeluxeMenus shop here.
You may also use my (much simpler) version here.