Broker the deal using the sellers FFL. Here how:
1) Tell the seller you and your FFL wants a copy of the sellers FFL complete with name address and phone number.
2) When you get #1, call that FFL (speak to the person who actually does the transfer and get their name) and explain what you want them to do. You want the FFL to receive a USPS MO from you, payable to seller and tell the FFL he is to turn the MO over to seller when he gives the gun to FFL for transfer/shipping. Offer to pay the FFL for his doing this on your dime. He is more likely to agree for a few extra bucks coming his way. Pay him with a separate USPS MO for the record, Get his agreement before sending funds. It's cheap insurance.
3) Go buy a USPS MO or two, if the dealer is getting paid for his trouble, take a picture of the USPS MO made out to the seller and send the picture to the seller with a note explaining he will get paid when he turns the gun over to his FFL. Or better yet call him directly.
4) If the seller balks at this arrangement the deal is off. Do not send any funds until the both the FFL and seller have agreed to your terms.
5) Take the USPS MO (both if one was going to the FFL) back to the UPSP and ask for a refund. They are obligated to give you your money back.
6) Done. You either get what you are buying or not. Yeah you might pay a bit more (broker fee if any) but you will know if the deal is legit.