Bobs snail shell is a quest item, and you can only hold one of them. These are both intentional. Bob is no ordinary snail. And he has a quest.

He snail shells from his summons however are meant to be regular snails and regular shells, this will need fixing.

We should also improve the error message when trying to loot a second one of a kind item like bobs shell to prevent confusion

Thanks Geylang. Bob's own shell is meant to be one-of-a-kind, so you can only carry one. The shells from his little snails are ordinary Snail Shells; they wore an 'Unsellable' label by mistake, and so did every other Snail Shell. That label is gone and shops buy them again. Trying to pick up a second Bob's Snail Shell now tells you the item is unavailable for pick-up instead of claiming your inventory is full.