Icloud signed in but not locked?

Hi, I just bought an iPhone 4s off of ebay and the icloud is signed in under the previous owners apple id. I'm still however able to use the phone and download apps and everything. I don't know what I will be missing/losing if I don't return the phone and the old ID stays signed in to icloud

Added (1). It is wiped of everything but their contacts and the fact that they are signed into icloud. I'm asking if this will have any effect on me using the phone

It might be stolen as anyone selling it would have wiped it if it was theirs.