You can contact your local police department with the gun's serial number. Explain that you want to make sure that the firearm is not stolen before you buy it.
The serial number will be checked via what we call "Triple-I" (Interstate identification Index).
The check usually only takes a minute. If a gun has been stolen, AND if the serial number was included in the police report ANYWHERE in the United States, the gun will appear in the data base.
It doesn't cost a penny to do it, and most departments will readily assist you in doing this.
Hiding in plain sight...