Can't contact someone

So today I saw that someone bought my biomass, But I cant contact him to give the item to him. So how do I contact him?

Did you message him via Gameflip? Once a buyer bought your item, both of you must message each other to coordinate the transfer in-game (e.g. getting gamer tag or steam id, etc.)

If you’re not sure where to message him, go to your sold listing and click on it. There should be a messaging feature where you talk to the buyer.

Make sure you capture screenshot of the item transfer, in case the buyer dispute.