I do not know commercial law over there but the following may be worth a try. Write to Gunbroker and point out all the negative things this bloke has done. Then point out all your efforts to get them to delist this advertiser. Then suggest you are going to get the police to investigate them as they are knowingly aiding and abetting a crook unless they delist the person and stop allowing him to advertise, no matter what name he uses. maybe mention getting the FBI involved as the adds go US wide not just one state. Point out that with the current anti gun climate they will most likely jump at this. They are always looking for ways to close down firearm sales.

Don't necessarily put the complaint to police/FBI just make the mention of your intent.