I would suspect it is like baseball cards. You can have an assessed value, but it is not actually that value unless you can actually find a buyer willing to pay that. A dealer will definitely lowball you because the only way they can make a profit is to pay you less than it is worth so they can sell it for what it is worth.
That is the hard part about having any valuable collectibles. You really have nothing until you actually have the buyer that really wants what you have. Depending on the product, sometimes that is easy and sometimes it is darn near impossible. For example, I may have a mint condition Ernie Banks rookie card, but trying to find someone in Texas who wants it at full freight price is going to be darn near impossible. Beauty is in the eye of the beholder and that sort of stuff.
I used to dabble in selling and trading sports cards at card shows and it is a given that the dealer holds the edge. They will pay you 50-75% the worth or trade you something that they have that you want. But the amount they have invested in what they are trading to you is likely 50% or less but they will negotiate it as full price. So, I used to be able to get super cheap wax boxes of cards that would sell for $20 a box. I may have paid $6-8 for that. You may have a card you want to sell me that has a book value of $50, but I only want it if it's a good deal and I can resell it (or add it to my collection). So, I may offer you $30 for it. You may balk, so I offer two wax boxes valued at $40 and that sounds much better to you and maybe you take the trade. What you don't realize is that I really paid about $15 for the card, because the product I traded you was something I had almost no money into. Then, I can resell your card that I got for $40 and still come out ahead.
The other aspect is that the seller almost always overestimates the quality of their product while the buyer will almost always underestimate it. Grading services for cards can normalize that a bit, but it can get pricey to grade all of your cards. I suspect that grading coins is similar. You find an old beat-up coin and think you can retire early, only to find out it grades out at far less than what the book value that you read is. So you leave disappointed and have to continue slaving away at your job instead of retiring early.
The biggest rip-offs I have seen are at Half Price Books. I haven't been in a while, but people bring in 4 boxes of books and shop around while the clerk prepares a quote for them. They get called back to the desk and the clerk says, we only want this one box and we will give you a dollar for the entire lot of them...or three dollars store credit. I have witnessed this numerous times. They almost always take the deal because they don't want to carry the boxes back home. Then the store keeps all four boxes. Granted, someo f it is junk, but they will likely put it out for sale. I noticed when those stores initially opened, you could find some good deals. But the last time I went in one, I was thinking they should change the name to Three Quarters Priced Books."
Car dealerships are much the same way. They never tell you how much they have in a used car they are trying to unload. It may be surprisingly little.
It all just depends on who you can find that wants the product you have.