Ah, I guess we're trying to avoid the 'multiple questions' problem, right? [...]On the other hand, I would only use 'for' in this sentence. I use 'to' with 'fall prey'.
"...he is easy prey for clever salesmen."
"...he falls prey to clever salesmen."
Also, I have absolutely no problem with X being "easy prey to" Y. That said, I appreciate the logic of your argument regarding for vs to, Cypherpunk, which is making me question my gut reaction that "to" is fine. It would be interesting to see what others think...