I guess you got the hang of it already, but yes, you were right in the op. Some other punter has backed that bet at 140, so you take his stake. If it is Chelsea HT -- WW FT, and only that, then he wins, you payout, otherwise you keep the stake.
Other way to make money on betfair is not to pick winners, but to look for the value bets, and then trade in play.
But beware! there are thousands of bots out there trading on the shifts in markets, irrelevant of what the outcome might be, just cashing in on people reacting slowly to changes (eg if a player gets sent off, lay the team losing the player, and then buy back at a higher price in a few seconds when the market settles at its new price)
It is impossible, imo, to beat these people who have direct electronic access to the market (ie one-click betting) when you have to do it manually. And never listen to the betfair forums, lots of people on there giving intentionally wrong advice to try and change the value of their bets.
Good luck