I can't really offer advice, but can tell you about my own experiences with liquidating a stamp collection that included a lot of first day covers that were in the neighborhood of 20-50 years old.

What I found was that there wasn't much of a demand for them on eBay unless you bundle a group of them together, and then you still may be disappointed in what you get for them. The problem is that the number of stamp and FDC collectors has declined steadily over the years and the post office really flooded the market with new issues over the last few decades, so there are a lot of old stamps and FDC's finding their way into the marketplace as collectors die off or sell their collections.

When I looked at demand for stamp collections or loose stamps, it turned out to be more advantageous to take the unused stamps out of the books and use them for postage as I needed them. Kind of sad to take a collection apart that way, but people who get mail from you get to see some interesting stamps when your letter arrives.

Still, it would be a good idea to let a collector look over what you've got in case there is something rare included.