Do not send money to a first time poster, only buy if you can go and collect it and pay cash. If they make comments like the price will go up in an hour, they are trying to get you to buy quick and are very suspect.
Ask a mod to check for other accounts from the same ip addres.
Ask them for a photo of the gun with a peice of paper in the picture with your name on it, if they really have the gun then this should not be a problem.
If the gun is very cheap then don't send money, only buy if you can go and view.