Marketpress - "free price" or "set your price" plugin

Hello [and pardon my english for I'm french...],
I plan to set up a social network for artists (buddypress+multisite+marketpress) to sell music in some "ethic" way = artists set up a minimum price for their albums and buyers are free to give more.

I did not find any plugin to do this, nor any trick/hack in the forum.

Did I miss something ? Is it possible ?

Thanks a lot for your help.