Check the stamp to see if they have gum or not (as you can see the catalog calls out a 'no gum' variety). Also check to see if the stamps have been previously hinged. Both these variables will impact the market value and you will need to understand this to compare with 'sold' values.
Once you have this info, then do this...
1. Go to
ebay and search for stamp by catalog number in US stamps as shown here...

2. The scroll down and click on the filter names 'Sold Items' as shown here...

RW34 mint, never hinged with gum (highest catalog value) have been selling for $25-$35 each. No gum or hinged stamps sell for less.
Don