So just like me you try and replicate the issue you can't.
I honestly do not know anything that would cause issue by a user. For the buy it now to even appear the seller has to have entered there paypal email and everything else to my knowledge happens based on the actual ad id. Userid of the user buying has like to relation to the buying process.
I will look at your files if you want me to but without you or I being able to replicate the issue more info would be needed from a reliable user.