Part of the tension in collecting most anything is when or where to buy or sell.
Generally, at some time during their collecting lives, most collectors become dealers at some level.
Most collections only last for 20 or 30 years in any event. Some people are very retentive of everything, even to the point of using few words when speaking. Others don't develop such a personal attachment to things.
The usual trend in collecting is to become more focused over a period of time and that frequently causes collectors to sell or trade. I think this is a very good thing, since otherwise a collection of anything would be very hard to assemble. You can't collect what's not around!
Some people become so involved in a hobby that they become dealers. I think this is great since such individuals, if they're ethical, can guide and help new collectors and fine very rare things for the more experienced.
With the advent of the Internet, collecting has changed greatly-CR is just a small example of this. I think that without the net, and eBay in particular, collecting vintage bikes/parts would be much more difficult. Of course, like many changes, good is accompanied by problems.
I sometimes also lament having sold things I have in various collections-but then I just begin to dream of the new things I'm going to acquire!
George Hollenberg MD
CT, USA