First off, let me say I have no experience in this area, but while reading this thread, I had a thought -- could you use a technique similar to the ones used to combat votebots? Since it seems you want your form to interface with a human and not a computer script/program, how about generating a confirmation image which must be reentered by the purchaser? I can't imagine the other site will want to hire people to sit around and reenter images as your product is purchased.

merlyn has some details here (Even though jcwren was able to get around it; see A little fun with merlyn *Smiles*)

Update

Silly me, got a snack and realized they will just pass the image to the end user, capture the entered input, and forward it back to your form...