Every trailer has a plethora of parts and accessories. All components do not work in unison. Some are more important than others. You do not need to consider all parts as quintessential and hence stock up on trailer spares. You would only need trailer spares for those parts and accessories that play a pivotal role in its function. Do not forget safety. Some parts or accessories do not necessarily contribute anything to the handling of the trailer, but they are a legal requisite for safety on the road.
You will need to buy chassis parts, brakes and brake pads, brackets, mudguards, wheels and tyres, electronic trailer stabilization, attachment parts, prop stands, jockeys and accessories, anti-theft gear, light boards and lights, rope winches, loading ramp, tool and storage boxes, chains, chrome fittings, graphics and decals, mirrors and safety equipment among others. You would need the toolbox to be well stocked, so you have tyre pressure monitoring equipment and other vital systems. These do not have to be deemed as trailer spares since they would not suffer wear and tear like the actual components in use all the time when you are on the road and even when you have parked or stored your trailer in the open. Exposure to the natural elements causes a certain level of weathering.
The parts that you should consider for trailer spares are the functional components that are integral to everything from hitching it to the tow vehicle to its optimum operation on the road. Trailer light boards are quintessential, but you may still choose to respond to a broken piece than have trailer spares for the same. What you cannot compromise on are the likes of trailer couplings and brakes among others. These are integral to the hooking and moving of the trailer. You cannot tow a trailer that has damaged couplings. You cannot be safe with a trailer that has completely worn out or malfunctioning brakes.
At times it is not just the whole part but a component of it that needs replacement. For instance, the damper in trailer couplings may have to be changed. The wheels may be fine but the tube in the tyres may need to be changed. The brakes may be working well but the pads might have worn out. You would need to replace the pads, the damper or any such component in a part that is creating problems. Such items should be treated as trailer spares, so you can continue driving or embark on your trip without any delay.