Sorry, but because you are only 13, he might have been wary of making a deal with you as opposed to making a deal with your father. I myself don't make business deals with minors. It's not that I don't think there are responsible minors out there.. or that I think you yourself are a irresponsible minor.. but legally, no contract can be called binding if the 2nd party is a minor.
But agreed with the rest.. first money talks. It's not the seller's problem to arrange for the buyer to be able to have the funds or have permissions to buy a snake. But luckily for you, there's a lot of snakes available, and I'm sure you'll be able to find another snake you want.
Good luck.