buy new, equip new, sell old, OOPS IT SOLD THE NEW STUFF!!

at least that's what I think happens, in any case it seems to point out the interface pays attention to which position in the inventory things are instead of what you're trying to sell actually is [internal] or actually displays [visible on UI]

Possibly limited to situations where the list of item doesn't change length because an old item is being replaced by equipping a new one.

It's really sad to have to do a week of money grinding all over again despite knowing of this bug and activaly trying to avoid it at the time. )-;

I know about the problem you experienced but it should've been fixed long ago I think? At least, a long time ago it used to happen all the time.

About selling an item by accident, there's an option to buy it back in the trade tab.