The Quadra at the lower left of the pile is an 800 (or possibly an 8100) -- 840avs have only one button, and it's up by the top of the machine.

The other thing I'd think about is whether the seller wants it all gone at once, or is willing to let individual pieces (such as just the 512k or just an iMac or Quadra) go. The description makes me think he's trying to get rid of all of it at once.