He said it was red. My guess would be some sort of tree viper since it had horns. Is it possible that the "horns" were scales that were drying out and peeling forward? I have seen that in snakes that get too dry. Im thinking if that was the case it could be an amazon tree boa. Good luck in your search!
-Edit: Just remembered the price tag said 1200, so that rules out an amazon tree boa unless the guy is nuts.
0.2 Bloods for Sale. Adult and juvinile. PM me for details.