I've sold 7 sets from the junkyard over the last several years, usually go for about $200 a set, so there is a market. There's one more, a super rusty, quarter panel smacked 351-2V auto drum brake Mach 1 with complete interior and I'm working to get the whole enchilada. The condition of the bucket seat tracks and bottoms are questionable but the doors are locked, so I'm on the outside of the glass looking in. Headliner is there, hard to ruin the bows.
My guess is that they go missing when cars are stripped and the restoration is delayed. Either that or due to my best friends - body shops. Those body shop dudes will pay super premium (over eBay) prices on whatever they need, because they are in a hurry and can pass it on to the customer.