some second-hand-sellers offer Fairphone 2 but explain that it has to be unlocked first - something you should be able to have done in a mobile phone store, they claim.
Sounds unlikely to me - but maybe someone can help.
Ask them for the IMEI numbers, a Fairphone 2 has 2 IMEI numbers.
I can check for you if it the IMEI numbers belong to a Fairphone.
(The Fairphone has 2 sim and each sim has its own IMEI number.)
Let them tell how they bought it, from Fairphone direct of a reseller (which one).
Then it easy to check with the resellers if the phone is locked and what it will cost to unlock it.
Sounds fishy, yes. Only yesterday we were made aware in our FB group that someone was trying to sell an FP2 in Berlin via ebay small ads with a similar claim. Follow Lidwien’s advice or ask Fairphone Support (who keep a list of FP2s reported as lost or stolen) if you get an IMEI from the seller, but I would expect that you will never get an IMEI from the seller.