Can I sell the new Iphone 4s I got on contract?

I'm thinking of getting a postpaid plan as it comes with an iphone 4s

Can I actually sell this Iphone the moment I receive it, will not open the box? I just want to sell it because of the high resale value and buy another phone

If you mean a contract phone, You can't legally sell it until your contract is over, that doesn't stop many people from doing so.

Yes you can sell the phone because carrier does not care what you do with your mobile you just have to pay the monthly bills.

Sure as long as you finish the contract. They don't care what phone you use as long as you pay the contract for 2 years.