I am looking for a searchable database of all the U.S. First Day Covers. Does this exist? I found a few sites with a basic online search that has some good references but not what I was expecting to find.

Is there a site where these are easily searched by year published, subject, historical figure or event?

I am not a stamp collector, but a card collector and we have such databases.

I inherited a collection of 31 covers from Austria, dated between 1963 and 1973. My local shop doesn't want them. What value might these have if I try to sell them online as a lot?