The stopwatch problem has the most simple solution

Make it so you can't sell a broken stopwatch but it can still be upgraded and then sold.

This consumes an entire slot of your inventory should you choose to take it just for shits and giggles and it is increadibly inefficient if you choose to upgrade the stopwatch item last. And it limits the build until really late into the game.

Ps. Can anyone check if selling a fully stacked ad tear item (was it muramana?) returns any gold