Not to worry!
Well, there is a level of trust on both sides. I find people are basically honest, but as already stated, get all the information possible on the gun that you are buying including photos. And then exchange personal information including references from previous transactions. I pay using a U S Postal money order. That and the fact that the firearm must go through a FFL guarantees delivery or else the U S Postal inspectors and BATF get involved. Most people don't want to get involved with the Feds and their unlimited resources. I have purchased and sold many guns over the internet with complete satisfaction.
If it wasn't for the NRA, the Second Amendment would already be gone! If you like guns/shooting/hunting please consider joining the NRA. NRA Benefactor Life Member