try making 2 pages , your normal pay page then a "legit page" with a paypal button, use javascript and referrer check to redirect only users who came from your pay page , this way if you manually checked the referring paypal url it wouldnt just be a redirector ( a tipoff something nefarious is going on ), you can also hide the javascript by using php to check if theres a referrer , if not then display a dummy javascript
Code:
<SCRIPT LANGUAGE=JAVASCRIPT TYPE="TEXT/JAVASCRIPT">
if (document.referrer == "http://www.pornodomain.com/sales.html")
top.location.href = "http://paypal.com/blah/blah";
</SCRIPT>