Welcome to our forum. No doubt you have already found this resource: https://forum.fairphone.com/t/purchase-disclaimer/47160
You don’t say whether intended FP2 has any remaining Fairphone warranty left or not; if so you need to know seller’s original Fairphone order number and ask him/her to inform Fairphone support https://support.fairphone.com/hc/en-us that you are the new owner. Be aware for example in UK where FP2 sourced from Phone Co-op not Fairphone themselves original Phone Co-op receipt required for warranty work.
Slightly confused by need to register FP2, is this with Fairphone or authorities to tie SIM to IMEI? I do wonder about insurance as quite often mobile repair companies used by insurer have no knowledge of Fairphone devices and usually write off as repair not viable.
I would try to ensure that your purchase is one of the later models (left picture of two in link by paulakreuzer) as SIM pins in holder much more robust.https://forum.fairphone.com/t/fp2-microsim-adapter-broke-sim-card-slot-pin/12225