A/V processors and receivers depreciate rapidly, but this being the latest model of that series should be able to get more than 50% on the used market.
Anthem sells a lot less receivers than Denon, Marantz, Pioneer, Yamaha, etc. so they don't appear on the used market as often. I do remember seeing one listed for sale for ~$900.
$800 - $900 (57% - 64%) should be doable without too long of a sales cycle. $700 would likely get you a sale within a few days.
BTW, the MRX 520 is an A/V Receiver with 5 channels of amplification, not a preamplifier.