Pretty sure you'd need to go through a sports signature authentication service. A credible one would verify his signature and then give you a certificate of authenticity. As long as it was reputable authentication service people would view it as a binding document.
You could obviously get a better deal on it by buying it before it was authenticated, but I don't think I would make that gamble, because even if your coworker thinks its real the person before him or even the person before him might actually be the scammer.
Just looking on E-Bay it doesn't appear Nate Robinson signatures themselves are worth a ton, however, a signed posters of his dunking performances with Dwight seem to be hot commodities.