Leveling fills your bags with grey items. Two add-ons take the vendor trip down to opening the window.
Dejunk
Dejunk sells junk at any merchant you open. You can extend "junk" with your own lists, for example old quest rewards you never use, and it can destroy items worth nothing so they stop taking space.
Leatrix Plus
Leatrix Plus has simpler switches for the same chores: sell grey items and repair automatically at merchants. If you already use it for other options, that may be all you need.
Pick one for selling
Both selling junk at the same merchant is harmless but pointless. Use Leatrix Plus if you want one add-on for many small fixes, Dejunk if you want control over exactly what gets sold.
Forever status
Both have Forever builds; their pages show the versions and dates.