I suspect I know who this is, & although I've not bought from him, I've spoken to him with a view to buying, & to people who've bought from him, & talked about him to one of his competitors, & everything has been favourable. That doesn't mean that he can't get something wrong, but it would incline me towards giving him the benefit of the doubt.
I'm not sure what I'd do in your situation, except talk to him - which it seems you've now done. I think I might tell him I wanted to build the bike, but on the basis that I was doing so to try it out before deciding whether to accept it, & see what he said. If he said OK, if I was unhappy with the frame after building it up & trying it out he'd replace it, I'd be happy to go ahead. Apart from anything else, this would suggest to me that he thinks I'd probably like it once I tried it. If his attitude was that by building it up I'd devalue it & he'd be unwilling to take it back, I might ask for a replacement straight away.
I'd hope for the first response.
