I have learned that unless the records are in mint, unplayed condition, they are worth surprisingly little to most collectors or stores. It's actually quite discouraging.
So I have adopted the philosophy of spreading the good sounds to a new generation. I trade in a stack of my LP duplicates of CDs, to a used music store in my city for store credit. I can shop "for free" for a while. I don't think about how little I am getting per album compared to how much I spent for it. Instead, I think of some fanatical new jazz fan who is just getting started, and buying as many albums as fast as he or she can afford them, who will delight in my albums at an inexpensive price.
The store owner tells me that there are such people, and that they tend to look at his "new stock" section almost daily for more jazz discoveries.