My rule of thumb with ANY used receiver, is to pay as little as possible, tyipically under $250, if I can help it. One, they're just "receivers". Two, you're buying used questionable history electronics, often out of warranty, and you don't want to be sitting on a $1000 paperweight when it explodes! Also, i'll keep eye out for a replacement after a year or so, and get my money back, and into the next piece.
It's all for sale eventually, and DEFINITELY degrading and depreciating as time goes one anyway.
Buyer beware, on any level..I always say