How to handle your players selling items they will need?

It's their problem - if they realize they sold the 'quest item', it's up to the players to figure out how to get out of the sticky spot. Strictly speaking, it's not your problem as the GM.

However, I would be more lenient for a more seat-of-the-pants kind of deal if they did do something like that - after letting them sweat a bit, of course, just as an object lession.

Basically, say that they needed the head of a sceptre as a key in order to get past a door of adamantine or something - but they've already sold the sceptre. And then they realize they need the head of the sceptre. Well, look at how it worked in Raiders of the Lost Ark - it became quite important that one particular Nazi agent had a severe burn on his hand in the shape of the head of the sceptre.

In that case, if the players were unable to come up with a plan for their characters to retrieve what they needed, I would be inclined to go along with any (plausible) plan that they came up with - but I might make it harder, or throw in a complication or two because they 'hacked' the problem instead of solving it 'properly'. And then I'd hope I would impart the importance of the next quest item better the next time.

/r/rpg Thread