In the timeline of things they forgot, Favorite is one of those things.
In my UI suggestions post, the accidental sell-all would be fixed by having a proper Buy Back panel,
much bigger than it is now. If memory overhead is a problem, they’d wipe it as soon as you leave town. If it ever becomes “not a problem”, the panel could simply hold the last XX items you sold. Beyond that, it’s the seller’s problem - as Blizzard could then say they gave us all the tools necessary to maximize efficiency & to keep our items safe.
( Currently it’s possible to accidentally sell the wrong item & make it irrecoverable because of limited slots… they should fix that. It should be at least the size of the backpack.)