I have bought 2 guns advertised on MGO. No problems, guns better than advertised, and met 2 nice guys that share 2A rights!
MGO often has some great deals. I've taken advantage of numerous opportunities there. Of course the good deals rarely last more than a few hours before getting bought. The vast majority of the ads there are over priced to the point of being laughable.
My best deal was trading a glock 22 that I paid $250 for in exchange for a mint condition HK USP45. Did the deal in the parking lot of a Harley dealer.
Done lots of transactions in parking lots of gas stations, party stores, and shopping centers. Never really gave it much thought as far as any danger.