Locked iPhone 4 — What specifically can I access?
The seller is most likely referring to Activation Lock, which will prevent you from using the iPhones entirely unless you know the Apple ID and password of the legitimate owners.

You will not be able to copy data onto those phones, or view anything on the screen other than the Activation Lock screen:



Pretty much the only use you could have for those iPhones would be for spare parts. However as @bjbk mentions, they were most likely stolen. I would advise you stay away from this merchandise.