You're making an assumption that he "doesn't know." I propose that he knows full well exactly how much he "could" be selling them for, but is choosing not to do it because he doesn't want anyone else selling any. As others said, eventually, the other JC's will give up and he'll have the market to himself and make back anything he might have made before but didn't. He would rather, if two gems sell, that he sells them both for less rather than he sells one and you sell one for full price.
You're looking at the short term, but he's looking at the long term.