You are right about looking out for bootleg copies. Several years ago I was interested in obtaining some music videos of a particular band I really liked (Heart). I bought a couple from guys and they turned out to be home-made copies where they put together one or two performances, by simply copying off the VHS they had taped off TV or the original VHS. The quality was poor, but still interesting, yet somewhat dubious as far as being legal!!!
If the copy being sold is off an original 16mm or other film it should be decent, but if its coming off of a VHS copy then its likely not going to be too nice!!! I guess its buyer beware when it comes to ebay!!!
Greg