Where can I sell stock photos as a minor?

You can always sell them through someone you trust who's not a minor, such as your parents. There just needs to be an adult who can sign a contract and can adopt the responsibility that the contract sets out, on your behalf.

Underage people can still earn money from their pursuits (think child actors), it's just for practical reasons there needs to be someone adult who can sign the contract for companies to do business with.