First Day covers for Famous American stamps range in Scott catalog value from $3-$10.
Yours are in excellent pristine condition and look essentially unaddressed which confers a premium.
Some have what look like pencil notations in the lower right corners which should not detract or could be easily removed.
The cachets are Artcraft, which are attractive and popular but also the most common.
Blocks of 4 on FDCs are worth about 1.5x the value for singles.
Remember however that stamps and covers usually sell at a substantial discount from catalog.
Some may sell for about 1/3 catalog.
For a very rough estimate, figure an average of about $2-3 retail per cover times the number of covers you have.
That is likely the maximum you could get for them, selling them yourself to collectors.
Most collectors on
ebay are looking for deals and they might sell for less.
Remember a dealer will not pay you retail price.
Hope this helps.