Buying Samsung Galaxy Duos S7562

Samsung Galaxy Duos S7562 used i am getting it in 7,000, condition is good and the set is around 4 months older. I am unable to figure it out that why the buyer is selling it at such a low cost, thats why i was thinking to check it properly. Please anyone could suggest me that what should i check before i buy it so i can be saved from getting conned

Check the phone invoice and then cross check it with the seller , then only you can verify it, if its not in name of seller then you need to be extra careful.