Why?

A needle gives the illusion that having a friend pierce you is safe. IT IS NOT.

These kids don't have the equipment to keep a needle sterile once its out of the package (before it's used). They don't have ANY training to pierce safely or correctly. Kids who buy needles are VERY likely to re-use them on friends for multiple piercings which is obviously disgusting.

Improperly placed piercings can cause SERIOUS damage. Tongue piercings can cause a large amount of bleeding, belly piercings can be mis-placed and grow out of the skin, septums can be misplaced and punch through the cartilage of the nose, I'm sure you can imagine the risk of nerve damage for any genital or nipple piercings (you would be amazed how many young teens want these piercings)

Badly fitted jewelry can cause just as many issues. I highly doubt that you sell labret studs or tongue rings that are long enough to accomodate the swelling of an initial piercing. If a teen were to put these pieces in, there is a very real chance that the jewelry could become embedded in the piercing or even cut off circulation and cause tissue necrosis. At the very least it will be incredibly painful and require an emergency trip to a piercer for help.

As a last problem, you're setting these kids up thinking that they've got a sterile piercing. So they think it won't get infected. SO WRONG. Now they have a brand new puncture wound and no idea how to take care of it. Sure they could have pierced themselves at home with a nail and it may have gotten infected, but since YOU offered them the supposedly safer needle, they didn't think it needed proper aftercare. A piercing that a teen does at home with a sewing needle is much more likely to get show to their parents if it turns green. Admitting that they did it with a needle from the state fair would mean even more trouble and probably a delay in getting treatment or help.

Many teens will avoid self-piercing if they don't have the proper equipment. That means that a large number of horrible piercings-gone-wrong are prevented just because needles are difficult to get.

You have a responsibility to REFUSE to sell piercing equipment to non-professionals.
References :
Piercing apprentice. Was relatively recently a teenager and have daily experience with them in relation to the subject of piercings.

It seems to me that if you sold a kid the equipment and he hurt himself with it, or a group of them used the same equipment on each other without sterilizing it properly, then you'd be in a bit of a bind if their parents came after you.

For one thing, that young children are likely to harm themselves with something like this is not only foreseeable, it's practically expected, and as someone who deals in body jewelry it could be argued you of all people would know this.

It's unlikely it would ever come to that, and I do agree with your reasoning that it's better to do it with proper tools than with whatever you have lying around, but it's just a matter of whether you want to expose yourself to that kind of liability or weight on your conscience.
References :