My friend supposedly bought an antminer s5 for $800 on amazon. Amazon is by far the best, with a whole range of used, factory new and second hand miners. Id recommend cutting from there.
It has amazing buyer protection, but you'll pay a hefty markup for that protection.
I sell on Amazon myself and they basically force you to cover any expenses Amazon incurs including charge backs, and of course if the unit gets damaged during shipment the customer is not obligated to return the item... Yeah that's not a joke. Read the terms of service.
It is good to buy at Amazon or Ebay, but it is not good for seller. Sellers can always charge more to compensate for the hash terms and conditions.