WTB threads on gun forums brings out the scammers in droves. Thoroughly vett, as much as possible, the seller. Ask a lot of questions, ask for specific things (pictures) that only the owner would have. An honest seller, especially for a high price gun or multiple guns is willing to do what you ask (if within reason). Ask for a phone number and talk to the person and ask questions that the seller will have to have the gun in hand to answer. If they give any kind of excuse why they don't have the gun handy, it's probably a scam.
I did this once for a specific GP100 I was buying, decent price, not low enough to trigger the gut reaction but still uncertain and wanted to make sure. Actually had several conversations (both email and a phone call) and he completely understood why I was cautious. He agreed to give it to his FFL and have his FFL call me to say he had it in hand and would ship when he got my FFL. We also set it up to have the USPS money order (made out to the seller) go to the FFL and the FFL would give it to the seller when he received the gun to ship to me. I had all the FFL's info so if it was a scam, the FFL would have been involved and the ATF would NOT turn a blind eye to that.
I'm in the business and a Ruger collector so I'm working with gun sellers and buyers pretty regularly and because of this I think I've developed the "gut instinct" for spotting a scam. If it doesn't seem right I do everything I can to either prove it's a scam or to prove it is a legit sale and so far I haven't been taken.
I have found that scammers don't want to answer many questions no matter how contacted, so if you ask for things, that are easily done, and they balk, it's probably a scam. If the scammer says they don't know how to do something via email, but they know how to post things for sale, especially with pictures, guess what?
Asking for signature required for a package doesn't do a thing to safeguard your money. You're sending money before they ship. They get your money - then thank you very much - goodbye - CLICK. You first have to be sure they actually have the item and then are going to ship the item when they get the payment.