Whether the business is legit or not, I can't say, but I have read in some forums that trades happened successfully. However, selling/bartering is clearly against T&C of all frequent flier programs.
IMHO, it's too risky to do as your accounts can be audited/invalidated/and you could end up blacklisted. I also save my miles/points so I can use them for aspirational trips for my family and myself, and they get used up much faster than I can earn them. So, I personally have no interest in selling my miles for cash and don't believe in taking unnecessary risk which clearly violates the T&C. Could be much more trouble than it's worth, IMHO.
Just my 2 cents.