I will make two quick points:
1) Same type of thing happened to me a while ago - sold a website, the buyer changed the pages, traffic dropped because of the changes + the buyer decided he didn't like the quality of the existing traffic. He filed a chargeback, Paypal didn't help me at all, and I lost about 60% of the funds (since he didn't fund all with his credit card).
2) You made sales because of forum postings - depending on how you explained your marketing methods to the buyer, it might be an issue. Its like me putting a link to a domain in my sig, making many posts on many forums and then selling the domain as having "traffic". The visits may be there, but only if the buyer continues the marketing. Of course, this means my domain was worthless as anyone can put links in their sig. I hope your website experienced some sales without you having to make promotional posts, otherwise it could be considered fraudulent.