IMHO- it all depends on the kid and the vibe you get from him, I agree with everyone else that you should indeed have him arrange the paper work with the parents, so that your legal bill of sale is not to a minor. But if it is the welfare of the animals that worries you, I would find out from the kid what he knows about their husbandry, and quiz him a little. You dont have to sell to him if you arent comfortable with his ability to handle the animals, and that doesnt make you a bad businessman, it makes you a good one, and exactly the kind that most people look for when purchasing pets or exotics to keep.
Personally, I have been keeping exotic animals since I was a kid, and I always took it very seriously. I am 25 yrs old now, but I can honestly say that when I was 13, I would have been able to handle it, and would have been as responsible about keeping them then as I would now. The only difference is, now I have more experience, but the only way to gain it is to keep the animals.
If you get a good vibe from the kid, and he is involved to the point where he is selling in shows, then it shouldnt be a big deal. And, if he really wants the animals, then it shouldnt be a big deal for him to get his parents to be on the paperwork.