Why dont you make it that if you want to trade you must be VAT registered if based in the UK and equivelent in ireland. That would cut out a lot of the dodgy people and would ensure that only legal business' were being dealt with. I know some people do it on the side to make a bit of cash but if you are trading you should be paying taxes etc. Then if there were any problems, you could get the law/trading standards involved. Sounds harsh but would make the traders totally above board and keep dodgy people away.
this rules out Zisco and WEPR who are good traders......