Have you talked to your parents about this?
Tell them that you're having a problem with this. That the snails are living things and you don't want to kill them as a way of solving the problem. explain that even snails shouldn't have to suffer just because they're not wanted especially when there ARE people who want them. Remind them that you paid money for the parents, who are the exact same kind of snail and people pay money for them every day.
Then tell them that you understand that your parents want to keep you safe by not having your info out on the internet. Ask them if they can please sell the snails for you. Either locally or online.
be reasonable, polite, and thought out.
My DD is 13, and if she pitches a fit, quite often my immediate response is NO. It doesn't matter if it's a justified fit, the whining triggers the primal part of my brain that is why some animals eat their young.
But if she comes to me calmly, and talks to me like a person, I always, always hear her out, and address her concern. My answer might still be no, but she'll understand why and know that I listened to her. And i would never, ever make her kill any animal under her care for convenience sake. I'm guessing that your parents didn't say anything like "Kill those snails by Friday or else!". I'm betting they probably just told you to deal with it some way that wasn't selling them online, and you're now at the end of your rope.
Any time you're at the end of your rope - talk to them. Parents are good for more then feeding you until you're 18 and giving you chores.