Of course you have no basis for your claim that he would have eventually found a buyer. This is a "manufactured criminal" and the police certainly got fed up with waiting for an actual market for his work to materialize. Entrapment is so insidious because when you turn a good guy, maybe there's no market for "good guy's evil works" anyway. (Who knows why, maybe he looks like an informant/plant/mole because he's a real artist and normal, genuine, good guy, who only got into 1 single crime - and no corresponding culture or connections, no other crimes, nothing - through an entrapment operation.)
Please read the article more carefully and you'll see exactly what I mean.