Actually, you can research
ebay "sold" listings to get an idea of what these stamps would go for on the secondary market. My check suggests that most #2624-29 sold in the range of $13-$18; whereas #3373-77 sold in the range of $11-$16.
Bottom line is that you might be able to get a little above face value to the right buyer, but that has to be weighed against the time and effort to sell them in this manner.