Can i sell my iphone 4s if its still under warrnaty

You can sell your iPhone at anytime, the warranty stays with the iPhone.

Apple does not buy iPhones, though they may be following a trade-in program at your local Apple store. That is normally only for applying a value to a new iPhone, not a buy-back.

If you want to sell it, you should go to Settings > General > Reset > Erase All Content and Settings which will restore it to factory clean condition with the current iOS version. If the sim card is registered to you it is best to keep it. If it is locked to a carrier, be sure to let the buyer know which carrier it is locked to so that person can get the appropriate sim card for the iPhone.

http://support.apple.com/kb/ht5661

If you want to change brands, probably the best del is o sell on something like eBay or Craigslist where you have more control over the price...I would keep the sim card, especially if I wanted to use the same carrier with the new phone.

If you are planning to use he same carrier, check and see if they have a trade-in program...they may offer a good deal with minimal hassle.

See this -> What to do before selling or giving away your iPhone, iPad, or iPod touch

(stevejobsfan0123 above already provided the link but I am reposting it)

You are welcome, and good luck.