I always buy stuff off of gunbroker.
I also usually use a postal money order. (Here's why)
If you use a postal money order and mail it through USPS, and the seller rips you off, it becomes "mail fraud" which is investigated by the federal government.
On top of that, once you file a report with gunbroker, they (gunbroker) goes after the seller.
In order to sell anything on gunbroker, you have to have a valid credit card on file. They simply charge your credit card if they can prove you ripped someone off. (It's in the fine print of their sellers rules)
Again, when it comes to firearms, buy from a FFL or insist that the seller ship through a FFL. This ounce of prevention is worth the world of cure.
I once saw a guy in line at HPD who bought a 870 from a private seller.
We were talking in the line at HPD and he was telling me how he got such a great deal.
As it turns out, HPD "runs" the serial number and the shotgun was reported stolen.
They confiscated the gun, pending their investigation.
I don't know the final result.