Maybe it could be like a book swap, where you each bring a book (or more maybe) and all are put in one spot and then folks select the book they want. Sort of a fair exchange. Might be there would be several spots, for different lengths of fabric?
I have no idea if this would work. In a book swap, there is usually a group of books and a person just takes one book and leaves one. It is on the honor system. I visualize 3 or four tables....one for one yd of fabric, one for 2 yards, etc. Folks could put their fabric on the appropriate tables and then select what they are trading for.
I would certainly enjoy being in a book swap, no matter how it was organized. Good luck with your project. I think you have a good idea.
Dina