There is no official method to buy and/or sell places. If another player offers to buy your place or sell you theirs, it's probably a scam. Do not give other players Robux or items in exchange for a place because there’s no official system which allows them to give you a place.
Roblox cannot enforce deals made between players outside of official systems. Never give out your personal information, including password, to someone promising to build a place with your account. If you give them your password they can also take all your items and Robux! Sharing passwords is a violation of the Terms of Service and Roblox Community Rules.
For more information on related matters, please see the following links: