Just offering some of the more obvious precautions.
1) never send money to a member with little history on this Forum.
2) if the seller has a substantial history of membership, research his Threads and Posts, to evaluate his/her legitimacy
You could still get burned, but the odds of being scammed can be diminished. You can also request a phone conversation to discuss the item, which can be helpful - but still no guarantees.
I won't so much as nibble on a seller without a year or longer activity, no matter how enticing the offer. In fact the best deals are the riskiest ones, in my opinion.
Without any built-in protections for recovery of your funds, a buyer is completely reliant on the integrity of the seller. If I have no feel for that Integrity based on a review of their prior posts on the Forum, then I conclude I have no basis for sending funds.
Because you're SOL should the seller fail to deliver, or delivers a ***.
I've not been burned on this Forum yet, following these basic principles. However I have passed on any number of classified sale Threads, where I judged the risk unacceptable.
Same principles apply to established gunsmiths - believe it or not. I think as hunters and shooters, we tend toward respecting the character of other hunters and shooters. Because we've by and large found other hunters and shooters to be good folk. However I had a lousy long drawn-out experience with an established gunsmith (not a gunsmith active on this Forum). You'ld think a gunsmith with 30 some years of prior practice should be trustworthy. All I can make of my experience is that sh_t can happen to any of us, unless the item is received in one hand at the same time money is handed over with the other hand.
Once you've taken reasonable efforts to ensure the legitimacy of seller/gunsmith and you still get burned, you'll never recklessly engage again. It's simply not worth the aggravation...