The thing with post-WW2 US postage stamps is that they were printed not just in the millions, but in the tens and hundreds of millions and were distributed all over the country. If you find ones that are mint never used anyone can go online and buy them wholesale for a percentage of their face value - usually in the 70 percent to 90 percent of their face value and you can still used them to pay postage so in theory there is an instant profit to buying them. The catch is that it takes time to put together enough of them to make the exact postage rate for today's mail and for most people the time to do it is money and makes this practice actually cost more than buying a new Forever stamp at your local post office. Used examples of post WW2 stamps typically go for a few pennies if you buy them wholesale or in quantity. On these post-WW2 stamps when you see dealers asking for higher prices than what I am describing it is because you are paying for the dealers' time and costs to stock these stamps and pay the overhead costs of their business rather than the actual value of the individual stamps. Exceptions exist but they are few and far between.
On First Day Covers (FDC) the situation is pretty much the same as there were (and to a lesser extent today still are) many retail companies that manufacture FDCs to sell to their customers. Artcraft, which you will note on many of yours, is perhaps the biggest example and so their FDCs are the most common of them all. These companies marketed their FDCs with the implied hope that a person would be investing their money in a future collectable and large numbers of people were enticed into buying them. There are many companies that used this marketing strategy back then and still do today, not only in FDCs but in all sorts of collectables.
In your dad's large accumulation of FDCs - you might want to rummage through them and see if there are any from the 1930s or earlier. Those are a mixed bag of values, but there tend to some that do have more value than the value of the time to sell them, and some of them can be significantly more.
Again, my encouragement would be to keep your dad's FDC accumulation as a memory of him, but if you really do want to just sell them my suggestion is to sell the books of them individually or in small groupings of similar dates or FDC makers, with perhaps something like 100 FDCs in each lot. Take some clear, sharply focussed photos and put them on
ebay with a starting bid of 99 cents and set a shipping cost of what it will cost you to mail them by ordinary US Mail in a large padded envelope or small box - do not try to make a profit on "shipping and handling" as that really turns off many potential bidders.