How to check if an iPhone is original before buying?

There is no means to remotely verify serial number or IMEI, nor Activation Lock status, nor to avoid a post-sale addition to an IMEI blacklist as at least one scam involves.

The seller must be trusted, or the purchase otherwise insured. Or you treat the purchase as speculative, and risk the entire purchase price, of course.

Background: The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community

iphonime wrote:

so there is no way to see if i have the IME number, if the phone is locked or not ?

The carriers can check, and you may certainly choose to waste your time with a check.

The industry trade group CTIA references the following service: https://stolenphonechecker.org/spc/

A blacklisted IMEI is not the only thing that can prevent you from using the iPhone.

And an IMEI block can be added after you purchase.

And fraudulent sellers can lie about the IMEI, too.

You need to either trust the seller, or assume that the purchase is entirely speculative.

Again: The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community

PS: Don’t buy anything older than iPhone 8, if you want to run current iOS software. Don’t buy iPhone 6 or older, if you want to run common current apps.

Buy it directly from Apple or from an authorized retailer or your carrier.