If you have a club interested in a player, then its a good time to sell.
Because there is major interest, your chances of getting a good price is high.
(If you try to sell when there's no interest, it's a struggle.)
So negotiate and you should get a price close to the maximum of what their value range is.
You may realise that a player who you thought was unsellable and will ruin your team if he goes, actually is replaceable, usually by someone cheaper with more potential or better attributes. You just have to find that replacement.