I've used these sites a couple of times in the past 12 months with no issues. I would personally stick to using the major companies and avoid all others. Main ones are:
Seatwave (part of ticket master)
Viagogo
Stubhub
Avoid a company called Ticketbis. My Dad and my mate got tickets for el Classico through them last year, and the ticket never materialised for my mate until the day of the match, 2 hours prior. And this was only as a result of shaming them online via twitter. My dad's ticket on the other hand was dodgy and he got bumped and he never seen the match! Worse still, ticketbis refused to refund him, claiming he required "proof of refusal" (WTF?!) He did however, months later, manage to recoup his money through his credit card company though.
I think some of these companies are at it and are happy to sell on tickets they haven't actually got someone willing to sell yet. FWIW I've used Seatwave and viagogo and found them to be fine. Prices are a rip-off though!