If they're not yours, as in you made them, I think it might be illegal or copyright something or other to sell them. Not sure though.
If you tried to do it privately its possible, but if you were to do it openly I imagine there'd be a lot of pissed off producers.

Sole has a good point - if you know the producers you bought them from, and they're available to talk to, and it's not too much of a daunting task (like you have a huge collection from 50 different producers) ask them individually if they mind you selling their clips. Then you'll know for sure.