These are really rare and this is the cheapest I've seen. Generally the 8 core models go for about this price. Only catch is that they've been delidded and you'll need to supply your own heatsink. I made an offer and they countered at $475, but I bet the moment everyone here starts putting in offers, they'll just sell for asking price.
Supermicro X10SDV-16C-TLN4F+ Motherboard Mini-ITX Intel Xeon D-1587 - AM | eBay
Supermicro X10SDV-16C-TLN4F+ Motherboard Mini-ITX Intel Xeon D-1587 - AM | eBay