Yeah, it really depends on your location. Some states outlaw the .50 BMG outright (CA, IL, NY, I'm sure a few others), but most states have no problem with it.
Some states don't care about face-to-face transactions. Some require you to call in to verify the person is allowed to own firearms, some require that you transfer through an FFL.
Lastly, $1200 is awful cheap for a .50BMG--"too good to be true". New, they're close to $10k, and guns don't depreciate like most things. If I saw that deal, and it was legit, I'd be all over it, not because I need such a gun, but because I could turn around and flip it at a several-thousand-dollar profit. Although it'd be tempting to keep it for a couple months to see how many marshmallow peeps it could penetrate...