It's a bit more complicated than that. Players sales (less any unamortised amount) go down as a profit in the year they are sold. Player purchases are spread over the term of the contract.
So sell a youth player for £25m and immediately replace him with a £25m player on a five year contract and the effect of that deal on profit/loss will be - year 1 +£20m, year 2 -£5m, year 3 -£5m, year 4 -£5m, year 5 -£5m.
When we get around to year 4 and 5 of that deal, the initial profit is no longer considered for FFP purchases. So the effect of the deal for the last two years is a nett loss of £10m (remember the limit by then will be around £25m).
But I wasn't thinking so much of player sales when I was talking about affordability. I look at things like Spurs a few years back. They raised £17m in a rights issue, which they invested in the playing squad. They got the likes of Dawson, Reid etc. They could afford it, and in the long run I think it probably was money well spent. But they couldn't do that nowadays without taking the loss on the balance sheet. Or say Southampton had £1bn in the bank. They can afford to buy a few hundred million worth of players, but they wouldn't be able to without facing punishment. Or say Arsenal make a few hundred million selling property. They could afford to invest that profit, but wouldn't be able to.