you should always replace axle sets of brakes, shocks/springs, suspension bushes, even tyres in my opinion.
you cant even buy single brake discs usually..... theyre only sold in pairs for good reason!!!
even if your selling a car.... if you fit new parts, it then becomes a selling point. it might not be worth much more, but a bit of investment in what your selling can only add to its value.
think of selling a house.... you decorate it and fit a brand new kitchen.... your chances of getting what you want for it improve dramatically.