I am a 15 year old amateur photographer, and I am looking to sell my photos online. Are there any legal limitations to this? Or am I allowed to sell photos without any legal trouble?
You are fifteen, so if your parents don't allow you to sell photos, then you are not allowed to do it. And it might be that if you tried to sell photos of an adult nature, that there might be some law that prevents you from handling such photos. I'll assume that is not the case, you have permission of your parents (or guardian in general), and there's nothing dodgy about the photos. And we assume you do nothing that would get a 25 year old into trouble doing the same thing (like cheating customers, selling copies of copyrighted photos without permission, etc.).
As a fifteen year old, in most places in the world, including the USA, you can enter contracts. The difference is that to protect minors, such contracts are voidable by the minor themselves, or by their guardians. But not voidable by the other person in the contract. So if you sell a photo to me (an adult), that's a contract, and you or your guardian can void that contract, but I can't.
So you can enter contracts just like an adult, and you won't get into legal trouble because of it. You might have problems, however. Let's say your parents hire me to design your website. We have a contract, I do the work, they pay my fee. If they change their mind after I spent two weeks work and dont' want the website, I'll sue them for the money. Now if you hired me and changed your mind, you could just void the contract and I lost two weeks work. But I'm aware of that, so this is too risky for me and I won't create your website. We can't have a contract saying that you can't void the contract, because you can void that contract as well.
So because you are not an adult, people will be careful entering contracts with you. The photo selling website may not accept an account for you, unless your parents sign for it. As a customer, I would be fine as long as you deliver the photos before I pay, so I'm never in a situation where I can lose money because you void the contract.
So: Legal trouble? No. Difficulty? Quite possibly yes.