PlayerAuctions

This enhancement supports

Spigot / Paper & forks only

It has great levels of customizability and flexibility, to the point where you can edit pretty much every aspect of every single menu. It also has compatibility with lots of other plugins and also an author who is always there to provide support.

The plugin does however lack in the open source department.

Spigot View AuctionHouse's official Spigot page for downloading the plugin.