How can I tell if a used iPhone is stolen based on its activation status?

I was looking at buying a used iPhone from a certain marketplace but when I ran the imei through a few imei checker websites it says that Find my is ON. the phone is question is listed as sealed and new and claimed to have never been activated.

I’ve encountered this for many such listings and was wondering why or how is that possible.

Are these Stolen phones or does Apple now sync the iCloud while making a purchase?