It is all too easy for your boot lid to become dented. From an accidental shunt or prang on the road through to an overzealous piece of reversing near a wall, there are plenty of opportunities for it to become damaged both on the road and when parked. Over time rust will eat away at the edges, the seals will go and wires and locks can become damaged. Often a simple visual inspect will confirm whether or not it needs changing.
Purchasing a good replacement boot lid should be no problem. However lids vary across models and makes and some have lights and wiring built into their construction, whilst others are simple covers for the rear of your car. Most have hydraulic systems that can be dealt with by even a novice mechanic, however due to the weight of the part it is always good to have an assistant.