I think sales on this forum are a bit old-school, in that the buyer and seller are relying a lot on trust. Plus, we're generally dealing in knives, which can have less than innocent purposes. Under the circumstances, I think you have slightly greater latitude to refuse sale. Maybe the buyer has a low post number, bad feedback, or just rubs you the wrong way.
That said, including things like "first I'll take it gets it" sort of binds you to that rule. I'd personally have a high barrier to go back on the sale - evidence that the law is being broken (customs fees, local knife laws, whatever), no communication, or someone who was a royal a-hole/troll.
So I'd probably sell to someone with a racist handle, unless he had a record of highly inflammatory posts on the forums.
SS1945 - cool reason behind the name. You'll find good guys here, but occasionally feathers get ruffled by a misunderstanding. None of this INFI stuff is any good, though. Lots of good knives in the other forums, though. (

Did I scare him off? Yay, more INFI for me!!!!)