Hmm, I do use the same thread in the bobbin as in the top, I have tried a number of needles with it too. I think it *might* be the machine, because I have other issues with it too (the only one I can think of right now is that I can't turn the hand-crank in reverse when the bobbin case is in it). It's a picky old thing.
I don't mind used at all, as long as it comes with the accessories and feet and stuff. And as long as it doesn't give me issues when I quilt!! Not sure I could find anything at a dealer in my price range, tho. I'll call around today and see what I can find.