I plan to buy Audi RS6 and i found one for 21900.What do you think is it fair price or scam?

It's always possible that it's a decent one but if it's too good to be true then,... If you can get a warranty and a thorough history then you can take the chance but if you read these forums a lot of very expensive things can go wrong and you want to make sure this guy's car isn't about to blow a tranny or needs an o2 sensor (engine pull).

Mine is for sale as of yesterday and there's one year left on the warranty. I'm located in chicago if you're interested. You'll pay more for mine but you won't have to worry about losing your shirt when it comes to repairs--at least for a year:) call me if you're interested. 630-712-9446