Tyler Ted,
Just my two cents worth but: I have a Scott's catalog that I use (look on
Amazon, you can find one a year or two old for relatively cheap or go to your library if you live in a semi-large city, it should have one). But, I also know that I'm "never" going to get catalog value for any stamp. So I also look at
ebay and check the "sold" listings. I base my relative value on an average of the two plus what "I" think it's worth.
Your duck stamps (you posted the pictures, right?) are spectacular (to me). If they were mine and I was going to sell them, I would first reach out to any hunters (or conservationists) I know and see if they are interested or know someone who would be. Try to find someone that is a member of your local Ducks Unlimited for the same purpose.
You'll realize more for your stamps (in my opinion) by avoiding
ebay and trying to work the local route.