The Bureau of Alcohol, Tobacco, Firearms and Explosives has issued a an advisory letter warning to all federal firearms licensees and firearms dealers. This letter warns that some scammers are using fraudulently altered Federal firearms licenses to sell, but never deliver firearms online. This is a must read for anyone dealing in firearms.
The ATF warns that a scam will usually start with an online ad. Respondents to the ad will receive a copy of a fake license. After they send a payment they never receive any firearms and the seller basically disappears.
The letter from the ATF outlines some ways to help you avoid the scam. It also gives you some contact information if you think your license has been used in this or other scams.