I wanted to bring this up too- I have had terrible experiences on buying films from ebay and fleamarkets. It almost always is badly faded, or has vinegar syndrome, or both. I now only buy my films from dealers, which surprisingly can be cheaper, but they are in business to sell a product that they stand by. The company I use most is Derann films, out of England. Somehow, their prices seem to always fall below what I see on ebay. They send out a list of used films via email once a month. Their website only lists the brand new (yes, they are printing new super 8 films in 2010) stuff, which costs several hundred British pounds for a full feature, but their used stuff is usually pretty cheap. I think I paid about $20 US for the Empire Strikes Back on super 8 for both parts 1&2. I have no affiliation with the company, other than as a satisfied customer.
http://www.derann.co.uk/
Another note on vinegar sydrome-I visited the home of a film collector, and when I entered the porch, I saw several film cans sitting on a bench by the door. I asked about that, and he said that was all the material he had that was going to vinegar syndrome-he wouldn't even let the stuff in the same house as his collection!