It it doesn't have the optional tank and its a 15" not a 20". If it has three different sets of brushes and is less than 2 years old I would suggest no more than $1,200.
If it only has the one set of brushes and is over 2 years old I would not pay more than about$800.
FYI I found one on-line being sold by a pawn shop in Vegas for $50 bucks.