Looks like you want to open clips4sale for webmasters
I see many people try to do some like this. But it's not work. Some Content Sellers offers this service as "partnership" for studios and photographers. But for conditions I wont sell me old stuff.
If you want get producers you must offer more % for them, and don't charging for anything in advance. Don't forget they give the stuff you want to make money with.
and better kick out the file-locker idea. It has not a good karma at all. Better you give any of your future customers a own interface to upload and admin the content. Registration only with verfied for professionals, with all IDs and docs, including from the content owner.