They have had 2/3 the value of the richest clubs consistently over a number of years. In the premier League , they have Man United, Man City, Arsenal and Chelsea above them in the money stakes. That's year on year, every year.
The fact that Liverpool have sold players for huge money is a testament to that. To buy a player, you have to sell a player.
It's weird seeing Man United supporters bitching about the Glazers leeching them, when their net spend on players over the last 5 years has been half a billion pounds, second only to Man City. In comparison, Liverpool's net spend has been just over 100mill over the same period, they're about 14th or 15th in the league in that regard.
If you have less money, you tend to be a little more careful with where your money goes. C'est la vie.