1) I run a search in my email for the ebay username. Since I don't delete ebay/paypal related emails, If I have ever done a transaction with him/her, I'll have their email address. Email address and Paypal account are one-in-the-same.

2) If it is chip/casino related, I send them a message through ebay asking if they are a member of CC>CC and/or a visitor of TheChipBoard.com. Regardless of their answer, you can tell them to go there and run a quick search by your member number which will give them your email address. Then request that they contact you directly.

3) (This works sometimes) - You can give them your email address in a different format. Like.... John(at)aol(dot)com.