This WoW Classic Addon automatically sells grey items and repairs whenever you speak to a merchant. Holding shift while doing so temporarily disables that functionality.
This addon is a minimalistic implementation for those who don't want to load kilobytes of libraries just to have very basic functionality. As of that there is no configuration it just works out of the box.
Note: as of Blizzard limitation this addon can only sell up to 12 items at once (so you have the option to buyback items you accidentally sold). If you have more than 12 items just close the merchant window and talk to the merchant again.