I think a database is a great idea but a huge undertaking. One thing I think you will find is a wide range of prices for the same thing. One day a NOS derailleur will go for $60 and the next week the eaxct same NOS derailleur will go for $90. Then, the seller does this every week, saturates the market and 4 weeks later and the derailleur now goes for $40.

It gets even more complicated with different levels of condition or with (it seems to me) very minor details that make an item "rare".

I enjoy watching prices on ebay and I bid on less than 0.1% of the items I watch. I tend to look at a particular region and era of bikes which comprises at most of 2% of the market. Also, at any one time, there is about 1000 items in US Vintage Parts alone. I like searching the UK and Italy sites and do some general worldwide searches. I skim Craigslist in Chicago/Madison/Milwaukee but have only been lucky with that once.

Again, a database would be very cool but very complicated. If someone wanted to set-up an Excel template, I would be happy to contribute the info I glean in my searches. What I don't know is if anyone else is interested in the stuff I watch.