The two main companies are Sporting Index and Spreadex, best to have a look around on their websites.
Basically you have a buy and sell price, the difference being the spread. You can lose or win a fair amount so be careful, today I bought at 35 and if I had got nilled then my liability would have been 35 times my stake.
Conversely the bet made up at 65 so I won 30 times my stake.
You can close out at anytime if you take the price offered, you could trade on the spreads long before the exchanges were invented.