I bought a used iphone. Can the seller install something that could hack my other devices?

I purchased a used iPhone 4S from an ebay seller & I am curious if they could install a device that could hack me or get my personal information. I did not set up phone service on it yet and have just been using it w/my wi-fi for basic functions. The reason I am curious is a few strange things have happened that I never before experienced on my main computer(MacBook Pro). First off, after I used the internet on the used iPhone a few times my main computer is now experiencing a pop up that says another "device" on the network is using my ip address. What does that mean? The next day i looked up a medical term on yahoo, because it was something I heard in a movie I was watching, and noticed that some of the choices that turned up had already been clicked on and read. I verified w/ my husband that he had not looked up this term and we both thought it was super odd. It looked as if someone had been researching getting some sort of medical certification. Why would that happen? Why would things I have never clicked on, on a yahoo search show up as if they had been? I hope I explained that correctly. If you do a Yahoo or Google search the options all show up in a bluish color and then after you have clicked and read it and go back it will show in a different color because you have already clicked on that site or article.(I am sure that you know this but I am just trying to explain what I am talking about) So would it be possible for the seller to do this and use my internet connection or hack my computer to try and steal our Paypal information? Is their a way I can check if someone has been using my IP or other areas on my computer? Any advice is very much appreciated. Thanks in advance for reading this.