My take on this:

Working condition with no warranty given is ironic isnt it especially both party didnt request to verify on the spot the functionality of the mobo. How to argue whos right whos wrong. Ask the buyer to send u a picture of the item and compare with ur earlier pic. Look for any bulging caps. Well unless it is there already and the buyers accept it then no warranty lo or it could be you failed to mention it.

At least do your part ask the seller to send back the item and do a 24hr burn in with photo using ur own components. He will send it back to you if he is genuine. Who knows maybe it is his components are faulty or not compatible. If still unstable both party are wrong cause u failed to prove to him its working and he also failed by accepting the product as it is.

Losses beared by both parties 50/50. IMO just by stating "in working condition" isnt enough to justify the none warranty term without any prove.