The gotcha is that now 18k+ members know about it. Took me about 30 seconds to find it... ;)
FWIW, I find it really hard to believe that someone would buy a circa $1.7k machine, use it once and then sell it for $300?* :-? Keep in mind that they run real hot and really need to be modified to perform well. Nevertheless, a competent tech. will be able to install some restrictors and Scace it to get it right.
Youd want to see it in the metal and Id want some proof that it isnt stolen and the bloke is looking for a quick buck.
The old mantra of if it sounds too good to be true may well apply here. Caveat emptor I say.