There is a national duck stamp collectors Society or Association out there. Google search it.
I found them when my dad was looking to downsize his collection. They have a dealer listing on their website. I know there are local dealers listed on their website.
Make it clear that you want an appraisal and be willing to pay for it. Selling the collection in part or whole to the same dealer who appraises it seems very risky to me. Big temptation to underpriced key parts of the collection in order to acquire a cheaply. Obviously a reputable dealer would be above that but…
Grouse