Have bought firearms via Gunbroker and Guns International with good results. Have sold via Armslist doing nothing but face-to-face transactions. I will always gravitate to a face-to-face but most of the stuff I like is unique enough it's rarely if ever close enough to my location.
The way I try to do it is to approach any buying opportunity with the attitude that I should pass it up. Then I do the research to reverse that decision. Had a very near disaster in the case of a $3000 shotgun I bought from a guy 1200 miles away in Maryland. Wasn't anything like represented but I was able to get my purchase price back and only lost the cost of return shipping.
Have had to (painfully) walk away from some guns that were exactly what I wanted at a decent price. But things like the payment option, poor pics, extra credit card fees, weak communications, suspicious street address, etc. etc. got me worried enough I blew off the opportunity.
It's my opinion there is no such thing as an online store, auto mechanic, gunsmith, bank, car dealer, credit card company, lawn mower or under arm deodorant that is either 100% perfect or 100% bad. (Well, maybe the deodorant......)
Anyway, it's on us to weigh the pros and cons. If an individual decides to write off a business completely you could be missing out on an opportunity down the road. I've got a nearly 40 year old squarebody Chevy at a shop right now getting some gorgeous work done. I've already missed one gigantic car show and it will not be ready for the next one either. But the guy does fantastic work at a reasonable price.
YMMV