I have someone trying to sell me an unlocked I-Phone 6 plus. They say it will work with Verizon. Will it?
best way to find out is to put a sim card in. if you've got an iPhone 5 or newer you can just swap the sim card yourself. otherwise you can take a trip to a Verizon store and they can help
Who was the carrier beforehand? A good way to test it is to bring your Verizon SIM card (assuming it's the same size) and put it in the phone.
Any iPhone 6+ made for use in the USA will work like a Verizon-sold iPhone 6+. Make sure before you buy the device that the ESN is clean and the device contract is no longer in effect. The same goes for any financing plan. If the phone us from Sprint, make sure it is unlocked first.
I haven't had to unlock one in awhile, but At&T also had locked iPhones. When in doubt, just bring a SIM card or have them meet you at Verizon.