The nice thing about Garters is the fact that as a rule they get along well together. If you don't want the babies be sure to have the same sex in the tank. I don't think releasing the babies to the wild is cruel. If the release site is well thought out and gives them a good chance at survival that's all you can do.
If Garters are looked at in your area the way they are in mine you would be wasting your money on an ad. I don't even know 5 people in my area who would take a Garter off my hands and give it a good home. Most people don't think of them as a pet, more like a pest. It's a shame. They don't know what they're missing.
Good luck.