MyIdentityIsntImportant Thanks! That's what I wanted to know - I've read a lot about buying second-hand phones in cash and away from your home etc etc in order to avoid tracking, but this doesn't seem to cover the case where the phone's IMEI is registered in some cellular provider's logs because of the previous owner - or in this case, just plainly visible on hardware.
That feels like equally incriminating data as just buying a phone with your credit card in a normal store - in both cases law enforcement can figure out you're in possesion of a phone with a certain IMEI, so I can't really see why it matters that they can track the "path" of ownership.
After all, as long as the phone remains uncracked thanks to Graphene OS, none of your data is exposed. I.e. - ownership/proof of possession doesn't matter.