Age group. Generation.

When a certain age group get money/opportunity their interest goes to what they coveted when not having money/opportunity (...when young).

Most collectors markets see this. Today and where I live 1910 to 1940 cars are becoming cheaper, pocket watches can be bought for peanuts, bikes from the same era is getting cheaper, etc. The guys dreaming of these are getting really old and actually dying. Next generation is not very interested as they have no connection to it = market for this era is down.

I am into 60ies to late 80ies stuff. Bikes, watches, etc. It is going to be the same.

Some very high end stuff always have its value and actually - very mundane "low end" also as no one has bothered to collect it and therefore it becomes rare after a long time.

I am sitting on quite a collection of watches that value wise are going up crazy at the moment. When I am to exit - who knows... might be in the same range as pocket watches today. Same with the bikes. As I am not collecting as an investment I could not care less.

If looking at it from an investment perspective - this is a good guide line:
Huge Supply + Huge Demand + High Public Awareness + High Prices = Potential Bubble
Low Supply + Low Demand + Low Public Awareness + Low Prices = Diamond in the Rough
Low Supply + High Demand + High Public Awareness + High Prices = Polished Diamond

It is written by a fellow watch collector that goes under the nick MSNwatch.

Add to this the generation factor and the forever important "condition, condition, condition-rule" it gets to be quite a guide line.

If viewing your hobby as an investment – keep worrying and ponder on the right time to exit. If not having an investor view on things - buy what you like, use it, flaunt it, be buried with it and do not bother.