Lost contact of previous iPhone owner

Dear Admin,

I recently bought iPhone 4 with full set (box, charger and so on included) from local online shop in Indonesia. I use it regularly due to there is no problem on that iPhone. Suddenly, the phone asking for restore and i restored it. Unfortunately, the seller never give me the Apple ID and password that attached on that phone. I dont know what Apple ID is for, what icloud is until i face this problem. I dont know if iPhone asking for icloud ID and password after restored. My friend suggests me to call the seller but until today his phone number cànt be reached. I've sent email/text to the seller, never be replied. I have no solution but sharing on this forum. Please help me. If you need the proof that i own the phone (not stolen etc), i can send you the image of the box with IMEI and SN of the phone. Once again, please help me. The phone only on welcome screen of activation, cant be activated.