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In Bismarck North Dakota there is only a verified dealers that can help you with the stamp collection:
Antique & Coin Exchange
722 Kirkwood Mall, Bismarck, ND 58504
"Antique & Coin Exchange" is specialized in both antique coins and stamps.
I would recommend to give them a call and see if they appraise the collection. Once you have the appraisal value, I would recommend to sell the collection on the internet so you can reach more potential buyers. You can also sell the lot locally, Craigslist is free, and a lot of collectors look at the ads on Craiglist.
Keep in mind if you sell the unit to a resell shop, the price will decrease 20-30% from market value.
You can sell the item on Etsy, it's like you put the item in a online shop. You may also check Amazon Market Place. Both places have good reputation, and you can easily sell the items for a good price.
Let me know if I can help you any further, otherwise, please, remember to rate my answer!