That price is pretty reasonable if it is in good condition. But the market is kinda small for 1366 boards, so you might just want to post on ebay.
Is that PSU a Corsair TX850, HX850, or AX850? I'm not familiar with the 850x. None of those three I listed would be incompatible with your motherboard. Why do you think it is "incompatible"? I haven't heard of that occurring so long as it's a modern motherboard or PSU (mobo since 2005, PSU since 2008).