Doomie has shown himself to regularly be highly overreactive in situations like this, and I personally find the seller's arguments quite feasible. Note, I am NOT saying who's right here.
However, the Big Book of Amiga Hardware states that KS 2.04 and ECS did not start until motherboard revision 6.3; The one in question was 6.2 and the Kickstart was one of Doommasters biggest complaints. ECS and KS2.04 were introduced together, so if the guy said the motherboard doesn't have the ECS Denise(and he did), why would it automatically come with KS 2.04??
Sounds like Doommaster got some factual information wrong, and then attacked this guy.