I don't agree with the Rags comparison. I actually think they're in the same boat as us in this instance.
I can't think of a single player we've sold since the takeover that we wanted to keep. Likewise for the Rags with the exception of Ronaldo who was sold for record money.
It's simple supply and demand. Because we are both at the top of the food chains we're not selling clubs. Players are only sold if they're deemed not good enough or we don't want them. We're effectively selling 'damaged' goods. Buying teams know we want to get rid so they immediately hold all the cards.
Wages are also an issue. If you look at a number of the players we're struggling to sell, they're on wages that far exceed their ability or reputation. It's a simple fact that the bigger clubs have more money than smaller ones and often the small clubs cannot afford to pay the higher wages and the player is unwilling to take a pay cut.
Often we want to sell more than the buyer wants to buy and clubs know this. We're left with the decision, either take a lower fee and get rid or be left with a player we have no use for on high wages. This gives us a reputation as soft, which is why other clubs take the piss. It's a vicious cycle.