Can I trade in an iPhone bought from an original owner for a new one with Apple?

The availability of the trade in program will vary from country to country.

Go to your country's store on the Apple web site and check.

You can not order a device from the US store or use the US trade in program if you're not in the US.

So long as it's not activation locked you should be able to.

You can pre-order the new iPhones from tomorrow, so just enter the details of the phone that you want to trade in when you pre order.