Firstly if something doesn't make sense it should ring alarm bells. He says "Haven't used it since I bought it because I didn't have a gpu" but both those CPU have integrated graphics.
Secondly if he's cutting corners on the test bench with incorrect plugs, no CPU fan, etc, then I'd say he's voided any onus on your part.
If you did feel inclined to offer a refund then I wouldn't pay it until you have received the motherboard back at his postage expense.