Ebay is a good assessor of market values in a collectors market, no doubt. I see from your signature that you are focussed on a certain aera and area - were the bikes you sold of the same kind?

My market observations, although i'm not selling much and didn't buy much recently:
The market behaves differently in different regions. The top stuff maintains their value, a Singer or Coppi-era Bianchi is not likely to drop in price anytime soon. If i noticed a price-drop, it was in the mid-level and low-level stuff - most people considered an old bicycle just "junk" 10 years ago, the awareness has risen and many a bike was dug out from dusty garages and basements. Fashion plays a part (also different from region to region) - here in Germany there is definitely a shift of interest towards earlier bikes pre 70.