Tickets offered for excessive prices, on all sorts of sites, regularly get cancelled. So the there's a real possibility that you might purchase such a cancelled ticket, which is no longer valid for entrance to the concert. For this reason, never buy a ticket on an auction site, on the street or from an untrustworthy ticket-site.
Is the concert sold out and you're still desperately looking for a ticket? Then leave your name on the waiting list found on the specific concert page at www.abconcerts.be or take a look on the ticket-trade page. With a little luck you may just be able to get hold of a ticket for your favourite band in an honest and safe manner.
AB also co-operates with www.ilovemyticket.be, where you can find everything you ever wanted to know about dangerous and trustworthy ticket-sites.