There are gun and knife dealers, but they will pay a only a percentage of the estimated value. They are, after all, running a business and they might hang on to some of the guns or knives for a while. At the last gun show I went to, there were two or three auction houses, their percentage varies but I think runs around 10 to 15% of the sale price. Since these places are online, they can post as many items as you want to give them. And then there are guys like my friend, a lawyer who bought several estates and sold them at gun shows or he sold collections on consignment. His fee was 10% + any expenses. He was a collector himself, and is very knowledgeable about Civil War and later swords and knives, and WW1 and II knives, clothing and webbing. Finding guys like him is very difficult - mostly by word of mouth. There is a huge knife show every year in, I think, Atlanta. It is called the Blade Show. If you can contact any of the organizers you might get leads to dealers, but again, people who buy large collections want to make money. If you talk to the Blade Show folks, ask if there are any knife auction companies, like the gun auction houses.
Another problem is valuing the collections. This takes a specific knowledge and short of buying books and doing it yourself, will take somebody a fair amount of time to research any knives that might be valuable. Prices for your knives will undoubtedly range from $10 up to possibly, a few thousand $ (don't laugh, I have three knives worth roughly a thou each - two pocket knives, one sheath knife).
You can start by making a list of your knives, then by googling pocket knives and knife dealers. As you go through their offerings you will start seeing what the market value is for some (hopefully many) of them. I'm less familiar with bb guns. Air rifles are a different issue, but there are experts out there for virtually anything collectible.
