Bought an iphone on ebay i think is faulty???

I bought an iphone on ebay and recieved it today. Supposedly it is in 'new' condition however i find that the bottom four buttons (phone, mail, safari, ipod) do not work!! The slide to unlock option is also very dodgy, does not work properly. What is going on, have i been given a dud? Is there any way to fix this? What do i do, i am so annoyed!