I bought an iphone from letgo, and it has a message Iphone disabled, connect to itunes What can i do? I can't find the seller either?

I bought an iphone from letgo, and it has a message Iphone disabled, connect to itunes What can i do? I can't find the seller either?

Added (1). Some more clarity: Letgo is an app where you meet a person and buy the item. He showed the iphone and it was out of battery, I took his work but once i charged it up, there was that message. I contacted letgo but they can't seem to help me. He has deleted the app itself

Better hope its not more than that. As far as what to do now, do as it says. Connect to iTunes.