Strange, maybe because of custom BIOS. I have X9 and X10 boards and they don't work with i5 and i7 CPUs, but i3 works. Also have tested non-ECC memory and the boards won't boot.
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Here's an example of X10 board working with i7 4790K but I'm not sure if it work the same on the X10SLH-N6-ST031.
The specific system on ebay does seem to use DDR3L ECC UDIMMs, however.