Here's a weird thing: if you don't have enough money to use the mirror, pressing the mirror key will sell an item (I haven't found out in which order the items are sold). However, once you have enough money, there is no way to quick-sell an item any more. Neither from outside the inventory nor from inside. Instead, you have to pick up an item (or long-press pick-up if it's a stack) and move it to the mirror. This is tedious and, I find, rather unnecessary, since there are still keys we could use for the quick sell action.

Could we get a quick-sell feature, please?

Totally agree XD
I just looked a bit in the Steam forum and we aren't alone to think and suggest that, so hopefully something will be made :)

And it's there, in the new update (version 1.8.10.1) :D

Hooray! =)