Betfair is one of the few sites which enables yout to bet against the outcome of an event or “laying” as it’s known in the industry.  So if you think that Chelsea haven’t got a chance of beating Charlton Athletic you can become a virtual bookmaker and offer odds against Chelsea winning.  If someone wants to accept the odds you’ve chosen they can then put their stake up against your odds.  If Chelsea win you lose the bet, the backer receives their stake back and you also lose money at whatever odds were accepted.  However if Chelsea lose you keep the backers £100 stake, which is effectively your profit.  One thing worth bearing in mind is that you stand to pay out quite a bit more than the backers stake depending on which odds were selected so choose wisely.