Of course they have to spend within their means, every football club on the planet is obligated to operate under those same limitations, but some clubs have a great deal more room to work with than others - Liverpool are one such club. They have invested just shy of £1 billion on transfer expenditure over the last decade, placing them 7th on the list of Europes highest spenders during that period. They were spending large amounts of money in the transfer market long before the sale of Coutinho.
Hearing Liverpool fans drone on about Netspend is frankly offensive. They support one of the most financially secure football clubs the world over, so secure in their wealth that reinvesting 100% of the Coutinho funds directly back into the transfer kitty was done so without consequence. There are few clubs in existence that can operate such a fashion, as a Spurs fan you can appreciate this more than most.
And some clubs have a great deal more room to work with than Liverpool, i.e City and United, one is ran by people who don't care how much is spent, and the other is a club which has revenue streams far exceeding those of Liverpool.
Liverpool have spent a lot, there's no doubt about it. But they've sold both Suarez and Coutinho in that time (two world class players) in order to fund much of that spending, which is simply something that clubs like City and United (who they compete with) really don't have to do. Liverpool even without sales can spend significant money, but not quite as much as the biggest spenders in the league.
Of course, I agree with all of that. Liverpool are in the very enviable position of being able to reinvest pretty much everything and having owners prepared to spend a bit more on top as well, they're in a great position. I don't think anybody is denying that, it's just also a fact that they're competing with City: a club who don't have to sell anybody, they can keep all their stars and add players on top of that. That's a significant advantage. They don't have to remotely care about net spend.
City don't sell De Bruyne to fund Laporte and Walker, they offer De Bruyne a new massive contract to keep away the biggest clubs, and then they bring in the players they need anyway.