Meeting someone that says they have a firearm for sale is always problematic. I will offer these tips as I will meet someone tomorrow about a *pony snake*.
Meet in a controlled place such as a friendly range that knows you. Never go to the sellers house! Do your homework on the hardware in question. Be prepared to have the proper purchase permit or a copy of your CCH permit (in this state anyway) to present to the person. Get a bill of sale from them! If at all possible have it witnessed by a third impartial party.
Let's face it. You are going to be carrying cash or a negotiable document.
Wish me luck!