By spotting them as a hobby before everyone else, Rob.
Gordon parts, or any of these odder light units/spares, do not need to be expensive. They become so if known to be fitted to a rare, or desirable, car. Otherwise you are paying the surcharge of a few previous owners of the bits who had the knowledge, or sense, that they were good to keep. Sadly that can really only be short circuited by finding the parts directly from fresh sources. That is what I used to enjoy doing. If you make a pastime out of doing that then it becomes clear you have to sell to make space for new stuff you want more than the old. Its clear to see how many a jumbler fell into trade. Once trade then your fiscal compass does alter a little.
I am not sure where we are now but there seems to be more people looking for fewer parts. The thing I do not understand is the demand for certain parts where the stored sets of parts must be many times the number of surviving cars to place them on. Most of these 'enthusiast' owners who have these extra parts tucked away are vehement that they are not trading, yet will hold out for 'the going rate' never having owned a car the units should be on. All rather strange really, unless they are trading in their spare time. Those campervans do cost a lot of money.
I find it interesting that the French are pretty secretive about their autojumbles. With good reason, or I think the vultures would descend and buy up all the shiny fridge bits and the price of what was common will become high, driving the whole market up beyond that comfort zone a lot of folk have where they potter with cars. This certainly happened in Germany, I was part of it, where you could find maybe 20 foreign buyers with up to £1,000s to invest in parts. It set me up in my chosen brands of cars and looking back I wish I had spent and bought more. No good news to newbies coming in now though. Germanys overstock of Microparts is pretty much at an end. Jumble prices are often at a level were you must need the part, if you can find it.
Having had those easier days, I cannot be bothered with Beaulieu now. Not been to Mannhiem for half a decade. I am pleased that folk still enjoy the ramble through other peoples cast offs but until I get into something I need parts for I am not motivated to put the sort of effort Richard does to go. And it is the way to do it, I am afraid. I bet even then Richard struggles to see everything on offer in September.