At the sniff of Facebook’s upcoming IPO, it seems that the trolls are creeping out from under the bridge in the hope of grabbing a piece of the action. A US law firm has announced that it is looking to put together a case against Facebook Credits, which it claims could violate antitrust laws.

As The Register reports, Newman DuWors has launched Stop Facebook Credits, a site it hopes will attract games publishers and virtual currency competitors who feel slighted by Facebook’s conduct.

The site lists a number of issues with Facebook Credits: Facebook’s exclusion of all other virtual currencies; a high 30% transaction fee (the site says that this is “double what competitive virtual-currency providers once charged for managing all aspects of in-game currency—not just transaction processing—before Facebook kicked them off its social-gaming platform.”), and Facebook’s insistence on like-for-like pricing where game publishers offer the same title on other platforms.

Newman DuWors’ Derek Newman told The Register that he hopes to build a class action lawsuit, or work with a large client that has “Nothing left to lose” after having had its business ruined by Facebook Credits. It’s offering ‘no win, no fee’ terms.

Facebook did not respond to The Register, instead directing them to its 2011 blog post on Credits. We’ve reached out to the company, although it’s worth noting that with an IPO forthcoming, it is severely limited in what it can say publicly due to the mandatory ‘quiet period’ following its S1 filing earlier this month.

In a related development, the New York Times broke news of Yahoo threatening the social giant over patents (some useful opinion on the matter here from Sarah Lacy).

While there are clearly legitimate questions to be asked over how Facebook handles its virtual currency, the timing of this latest threat reeks to us of people looking to make a quick buck (or several million) from a company that will soon raise at least $5bn.