That is a very good question, at launch it will be very basic and so it will probably be offer and acceptance only. Betfair is able to cross match bets across lots of users but they have a very complex platform to do this. So in your example if you enter the bet first with 200 SCR then someone will have to match that 200 on the other side. If the 100 SCR was placed first then you would only be able to take 100 SCR the other side and would need to place and additional 100 SCR bet which would sit there waiting to be matched. The other down side is that probably you will need to match total liability and the system will not be able to calculate and allow for the bets and lays you have made, unlike how Betfair will know if you made a bet on both sides and your liability is not the addition of the total liabilities. I expect these types of enhancements to come in later versions. That is my idea of how it will work as I am expecting it to be very basic at the start.