Is there a way to look up the IMEI to see if an iPhone I wanna buy is activation locked?

Not reliably.

Remember, there is nothing to stop a scammer from selling a phone, pocketing the money, then reporting that the phone was stolen to the carrier so the scammer can have the insurance company send him a new phone.

The scammer gets a new phone and he pockets the money from the sale of the phone, which is now useless.

NEVER buy a used phone unless you personally know the seller, and they have agreed to refund your money in full if you are not satisfied with the phone.

Nope, not reliably. There are companies that claim they can do it, but that does not mean it works. Not something I would count on when making a purchase of a used device.

You are always taking a gamble when purchasing a used device from an unknown seller and unable to return the device if you are not happy with the purchase. That is the risk you take and why it is not recommended.

I concur with Bob and Jim.

See here for more information:

The All Too Common SAD Reality of Buying … - Apple Community