If your looking to buy a kart for the first time, or even looking for some parts for your kart, consider buying from ebay, there is a brilliant selection of karts and accessories both new and used. Its a fantastic way to bag your self some bargains, just because the prices are cheaper certainly does not mean the quality is any less. Obviously if your spending a fair amount of money, its always advisable to go and view the item in question.
I would definitely recommend buying a second hand kart if its your first! Get to grips with the basics of karting, make the mistakes (spins, crashes) in an affordable kart, then once your comfortable invest in a new one.
Another great place to buy second hand karts and equipment from is karting.co.uk, They have a dedicated market place.