I think it`s a good idea to release the one you found, but the G.rosea(Chilean Rose Hair ) you are going to buy, is probably WC too.
G.rosea grow extremly slow, and petshops want to buy animals they can sell right away.
Can you post a picture of the one you cought? If it`s a male, it won`t live for long, and will serve the nature if you release it. If it`s a female, you might consider keeping it, but it`s up to you.
Regards
Robin