Best to be wary of used phones from strangers in general. Kijiji, craigslist, ebay, whatever. They could be blacklisted. Or broken in subtle ways. Older devices often have dying batteries. They could be carrier-locked.
Most sellers are legit, they're trying to sell their old phone, not trying to rip people off.
But many sellers are surprisingly unsavvy about phone tech they use every day - they have no clue what locking and unlocking really mean, they abuse their batteries, they manhandle and sully things in software/firmware which are better left undisturbed.
And some less ethical people looking to get rid of useless (or "lost"/stolen) junk they can't get working, take your money, and run.
Mert the seller. Put in your SIM card in the phone, test incoming and outgoing calls, test incoming and outgoing texts, test data on a random website (without signing into anything). If it doesn't all work properly then you don't even have to explain why you're not interested in buying.
