As stated, you'd be hard-pressed in finding anyone willing to pay for a large RTC when they're so readily available in the market. However, a show quality specimen of high color (bright reds/white) and no gill curl may go for $100 or so providing you happen upon someone looking to add to a monster tank already full of monster-size specimens. That's a rarity in itself.
Craigslist often features RTC from 6" to 2", from $50 to $200 in almost every corner of the USA. Far not all sellers state that they need proof that their fish will go into the right-sized home. I am rather sure they rarely get much for their fish or even find a buyer at all. Sure, some get lucky but it is rare.
It is no different from buying mammals, reptiles, birds, just to grow them out and sell them to make money... or at least having no intention to have them for the pet's and/or the owner's lifetime. The vast majority of pet-keepers would condemn such practice, I'd think. I think cat and dog owners would be appalled at that. Not that the majority is always right but here it feels it would be.