I started a thread recently wanting to buy links for one of my sites, and as expected my inbox was flooded. A lot of the PM's though were offering me links on sites with fake pagerank, and trying to take me for a fair chunk of money in return for an almost worthless link. To me this is a clear attempt at defrauding fellow members of this site. I just wanted to check though, am I ok to report such messages or is the site's attitude that it's completely a case of 'buyer beware' and they're not interested in such matters?
There is no rule against reporting whatever you feel needs to be reported. The report button is at the bottom on the left hand side. It is of course up to the discretion of the mods as to any action taken, or not taken as to your report.
Truthfully, the best thing to do is not buy links. Generally you are looking for a service that is more or less to spam/scam search engines. It's really not surprising that you get some fraudsters trying to sell you something since it's the nature if what you are trying to buy. It's like going to a seedy part of town looking to buy drugs and then being surprised that the people you are dealing with aren't exactly stand up people.