Things you shouldn't buy used
First things come to mind: underwear/lingerie....but i ain't that shallow... lol
Laptops: you are taking a chance when you buy any used computers, but the risks are much higher when you purchase something as prone to abuse and hardware problems as a laptop.
Exception: You are buying a refurbished unit with warranty.
Plasma TVs : This is something you'll definitely want a warranty and an extended warranty at that. Its gonna be a bitch (physically and on your wallet)to repair when the tv breaks down.
Exception: Its a super-duper great deal and you don't mind if it breaks down just when you bring it home
DVD players: You can used DVD's, but not the players. The laser is too fragile and integral part of the set(no laser = no fun) to take a chance on.
Excpetion: Its your buddy's and you've been using it all along...
 Helmets: They are meant to protect your head, DON'T compromise!
No Exceptions here...cost of injury or death is much higher (touch wood)
 Mattresses: Think of all the stuff you do on your own mattress.... now think of sleeping in your birthday suit on someone else's bed... EEWWWW...Buy NEW, after all, like the tv ads say, you can even get a free mobile phone for every purchase(!)
Exception: You like sleeping in somebody else's germs, bacteria and bodily fluids... urgh, i don't wanna think about it.... NEXT!
)Shoes: Poor fitting shoes can give you the whole array of problems: heel, shin, toe, knee and back pains. Don't buy shoes which have already been molded to someone else's feet. Go for shop and warehouse sales or factory outlets for shoes at lower prices, but brand new.
Exception: You are like me, a sneaker collector, that desperately needs a particular model/brand of shoe and can't wait to throw a fortune on a worn pair. :P