>Say they buy it for $20,000. Do you really think someone will derive $20,000 of profit from this before it's caught and patched by Facebook?

I can absolutely think of a situation that would make access to one person's facebook account worth way more than $20,000. Say, an unethical high-profile divorce lawyer fishing for information that would help during a multi-million dollar case. Or a political activist trying to dig up dirt on a candidate.