Shouldnt be too hard to implement with something like this into WordPress Simple Paypal Shopping Cart or similar and using a function to echo out the advertisers paypal email address into the paypal call, but i would be wary of doing it, from my point of view on a small scale classifieds site it could lead to some abuse by advertisers, you would need to monitor it very closely, you only need a couple of bad guys around and thats the credibility of your site gone.
