If I was selling that bike, I'd probably accept $400 at the lowest.

He states it has low mileage and if it has little or no wear and tear- if it's near-new- then I don't see why he would accept less than half the original price of the bike other than if he's desperate.

I don't see what you have to lose by offering $300, or $250, for that matter. It's just an email.