Most likely does not want to deal with two transactions. Just by the some of the things he said in his listing, I think he just wants to be done with selling them and not deal with a lot of people.
I get that impression too. If he feels that way though, he should put his price out there and say he's Firm on it. Those cars won't go cheap and by selling them as a set, he narrowed down the buyer pool to a small handful of people who have a safe full of disposable play money. I'd like to get one of these cars someday but if I had the money ready to buy, I probably wouldn't even call because I couldn't afford two at once.