If they don't respond, then the sale falls through, seller has to agree to sell.
I know clubs make all kinds of offers in reality, but they also can view a listing of players actually up for sale. And I really hate that it's only players close to level up too. As they are the very ones you want to hang on to :/ But, otherwise it would be randomly selected players and probably more annoying.
Just wish they could add in the ability to choose players to add to the Neg. list.