Pending Sales is an effort by Roblox to prevent trade scamming involving Robux. When purchasing an item, the creator must wait three days to earn their Robux. It was officially released on September 2018.
Pending Sales are used to hold your Robux from showing up in your account. It is used to help prevent scammers, and give roblox time to investigate if a scam has taken place. This is a security measure and is used on every sale made towards your account.