Just bought 2016 MBP and MBA CHEAP!!! Need advice

Hi. I work at a small computer store and recently a guy came in to get 2 2016 MBP and 1 2016 MBA erased and reinstalled claiming that he had forgotten the passwords and the user accounts were locked. First I PRAM'd them all then booted from a High Sierra clone and formatted the internal drives then cloned them with CCC. After I did the first then he offered to sell me the others CHEAP. Too cheap maybe. So far I have only reset the PRAM and reinstalled High Sierra. Is it okay in the event these didn't belong to him originally to sign into the App store? I'm looking to use these for personal use but want to be safe. I don't necessarily need to sign into any Apple services but it would be nice. Can these be tracked at the moment? Any help is appreciated!