Even if the phones are clean now, he can still report them stolen after he registers an equipment swap to your phone. I wouldn't do a trade like that (assuming I wanted one of the phones he has) unless it was done at the carrier's store, and the change of ownership of all 3 phones was registered with the carrier before the store associate handed the phones back. (Once you're the registered owner of the G3 and Z3, he can't report anything about them - he's not the owner.)
Whether you think it's a good trade or not is up to you. I like a lot of features of each of the three phones, so it would be a tough decision for me.