Well, I guess you haven't read enough threads on FT about the accounts being audited and frozen. Otherwise you wouldn't have come to such conclusions. As far as I know, there are threads about sellers getting caught in AA, Delta, NW & UA fora every single year.

I agree the fear of being caught is not the serious part. What's serious is the consequences. If frozen accounts and losing miles are within your comfort zone, please go ahead. I recall reading a thread that someone was barred from flying Delta due to the act of selling lots of Skymiles. I believe there's some legal action involved and he also received a letter from the legal department indicating he's officially on Delta's "no-fly" list. Maybe someone can dig up that thread for your reading pleasure.

My point is there's a range of consequences when one sells miles via the channels with no affiliation with FFPs (craigslist and ebay are two perfect examples). If you think you can handle the consequences, nobody can stop you.

FWIW, not everyone who gets caught is willing to go public and post on FT. I am sure there are quite a few untold stories out there.