Consumer Redressal

Dear Sir / Madam, I had a horrible experience with one of the online shopping portal. They sent me a used / second hand mobile phone, in lieu of a brand un-packed phone. The phone that I received has close to 115 addresses in the contact listing. I am still pursuing with them for a amicable solution. They are asking me to send back the phone, and upon receipt they will dispatch the new phone. Can I believe them? I would like to pursue a formal legal action against them, but would like to have your inputs as well before I decide anything. Thanks in advance. Kind Regards, Protoya Mitra