I use betfair and there is something that I don't understand...
Is it possible to bet in a match (for exemple Janko vs Ilia) on both players? Like I put 2$ on Ilian , and 2 $ on janko?
Because it seems possible... But if that happened, I would be always winning something so it's weird...
Yes, it is possible to back both players and make a risk free profit. When looking at a market on betfair, if the backing % (the % number on the "back" side of the market) is below 100%, then this can be done (for a certain amount of money). Conversely, when the laying % is above 100%, this also means that there is risk free profit to be made.
Alternatively, you can create your own "book" of above 100% by using different bookies/exchanges, by placing bets/having bets matched at different times due to price fluctuations - which is the main aim of most live bettors.
Most people understand that backing each player in a match at 2.10 can result in a risk free profit, whilst they do not realise that backing one player in a match at 3.10 and the other at 1.60 is more or less the same thing.
The easiest way to try and calculate whether it is possible to make a risk free profit in your head, is to take the (decimal) odds of each player, subtract 1 (so that you are left with net odds), and multiply the two together.
If the number is higher than 1, there is free profit to be made.
If the number is equal to 1, you shall neither win nor lose money (be wary of commissions).
If the number is lower than 1, you will be taking on some form of risk.
The following is a brief list of "par" odds. If you can get better than this in the same market, by all means take it:
Betfair operates by taking commission on a percentage of your net winnings after the event, whilst most bookies effectively take their commission out by giving you lower odds to being with. You will notice that bookies will never (deliberately) offer the above listed odds pairings.