Betfair is one of the largest, if not the largest, online betting exchange. It is the trading name of The Sporting Exchange Ltd, a registered limited liability company in England and Wales. It was founded in 1999 and holds a bookmakers permit in England and Wales. They utilitse secure servers and financial transactions are processed by DataCash and The Royal Bank of Scotland plc.
It has the most liquid market amongst the online betting exchanges and so it is most likely that your bet will be matched. So you can start being a bookie as soon as you sign up. The operating margins for an online betting exchange are much smaller and hence the commission you pay starts at only 5% and reduces depending on how much you use Betfair. Remember you only pay commission on your winning bets. The only other costs are the pass through costs for deposit tranactions that are charged by their bank.