Cash app?

Hi. I usually offer shipping but only through offerup. Just this morning someone wanted to pay me with a cash app. I've never heard of this and I don't know if offerup even allows using it. Can someone guide me as to if it's legit. Thanks.
Community Helper
Using cash app is going off app. If shipping please refrain from going off app to get paid or paying. If off app, no one has any protections.