I've been to a computer fair near Newcastle once and found everything is a lot more prefessional then I expected. There was only one stall selling illeagal copied CDs while all the others sold mainly genuine hardware. My friend has bought a motherboard and processor from there. They both came with a receipt and he has had no problems with them.
Don't compare a computer fair with a car boot sale, they are nothing alike. It is run very professionaly (well the one I went to was anyway)