Is it safe for me to buy a chip unlocked iPhone?

If you mean a carrier locked phone, ONLY buy an unlocked one.

For a new phone only Apple sells unlocked phones in the US. The rules are different in other countries; for example, in the EU and UK all new phones must be sold unlocked.

Be VERY CAREFUL buying a used phone, there are a lot of scams and sellers of stolen phones→The All Too Common SAD Reality of Buying a Used iPad/iPhone

Only buy a used phone from someone you know, and can contact after the sale.