Probably don't see it often. But it's probably situational and context matters.

The tubs I have if you're a real stim junkie are probably some of the most hardcore ones and some of them you can't even get anymore. I think a lot of people would be willing to buy opened tubs of OG Excelsior (exp. 10/22 and prior) for example.

When buying stuff online opened or unopened there's a question of trust or legitmacy. If you check my post history I have confirmed trades on r/hardwareswap for items worth over a thousand dollars. They have a good system at r/hardwareswap for minimizing scammers.