Cool visuals - this is exactly the stuff I was into in my teen years (I was collection stamps of the colonies from all over the world, from the beginning of the XX century to WW2).
Read your opinion with a huge intrest!
What I can say?
'Buy for cheap, sell at max' is _the basic and main law of retail trade', so... one cannot be frustrated at poor collectioneers for that.

And.... speaking about the 'invisible but wise Hand of the Market' that 'determines the true price' ... well, see, in the 2000s I was buying a lot of old papers on eBay; not as old as your letters and stamps, literally stuff from the 90s, newspaper and magazine clippings, promotional photographs etc. At least in this narrow and certain field, I have got an objective picture in its dynamics.

Here it is: b.s.! looking at the current eBay prices, I just smile and think something like this: “Crazy people!”... Yes, everyone wants easy money in exchange for nothing... its very easy to understand that.