This is a really tough question. In my experience, the thing to do is develop a relationship with a person/store who sells books, and then tell them you have something you’d like to sell. That way, you know it’s someone you trust, and they won’t undercut you, and they’ll buy it from you whether they need it or not.
This last part is key. The value of an old book is whatever the buyer bestows on it. Any antiquarian bookseller will have his own personal “wish list”, and if your book is not on it, you may not be able to sell it, or you may sell it for a great deal less than you expected.
I have two contacts in my city who both want different things. Some parts of my collection are worthless to one, yet valuable to the other, and vice versa. This is a long game. It has never occurred to me to try to sell outside of my city, because I do have options here. And, of course, shipping is a big risk for this kind of sale, because condition is so important.
So, I wish I could give you some actual advice, but I’m afraid I’m only going to suggest that you start to think of the selling process a bit differently. Find the used bookstores in your city, and figure out what they sell. Most used bookstores will have a few “old’ books, but regardless of how they display them or what they’re charging, that isn’t their business, and they are only trying to turn a high profit on things that look rare. You want to find the stores where the owners are highly invested in the selection they offer, and go in and have conversations with them (or with the buyer, which can be as good). They are probably going to be snobs about it, but it does come from a place of love and respect… for the books. Even if these people won’t buy your book (or won’t pay much for it), they can likely point you to someone who will. But it takes an investment of time and trust on your part. This isn’t like selling shoes.
You say that you’re in contact with an association of bookstores… this basically gives you a list of stores that you can approach. Start there. But a cold call is always going to net you less than a personal relationship will.
TL;DR: Be aware that your expectations are probably too high in terms of how much you can get for your book and how quickly you can sell it. If you have enough patience, you may be able to get a reasonable price for whatever it is.