Well, I'm sorry but that's only to be expected. It's obvious that the only people who are interested at all in vintage HP calculators are the old breed, the ones like us that found them amazing and almost out of reach in our youth, then have become old enough and wealthy enough to reminisce about them and get as many as we care to, so that we can at last have what we couldn't before, and we can indulge in some fond remembrances while admiring their quality.
These traits are not inherited by the new generations, even our own children, which actually couldn't care less about old calculators, not to mention "RPN". At most, they'll be mildly interested in daddy's antiques, but only because daddy seems to have a crush on them calculators and insists on proselitizing to them children with that RPN thing. At the very best, they'll think these machines are moderately 'cool', and perhaps they might be 'coerced' into using one at school, for a while ... But they'll feel nowhere near the utter passion we had for them, so, in time, they'll mostly forget about them completely, just bothering to eventually put them on eBay, after their old man's passed away, to see if someone will take them and provide some good money.
This being so, and with the recent proliferation of extremely good emulators/simulators, such as Nonpareil, Emu71, Emu42, V41, Free42, HP-71X, etc., which even in some cases will perfectly run in the smallest PDA devices, and which in all cases are many orders of magnitude faster and RAM-capable than the physical machines, plus the added convenience of running in a PC-like environment, means that even old-timers as ourselves might find it more convenient to use the emulator than the actual calculator. Just as an example, I find Thomas Okken's Free42 running on a small PDA much more convenient, useful, and easy to use than a real, physical HP42S.
The obvious predicted trend is then: vintage HP calcs have probably reached their summit at eBay, and from now on their value will slowly but steadily decrease to the point of finding no takers at all unless the prices are extremely low or some real curator wants that particular model for a real, full-fledged museum. And there aren't that many.
Myself, I'm not worried at all. I only have a few HP calculators of a very few models, strictly the ones I like. They're for my own pleasure and, except for some redundant ones, will never be for sale, so I don't care at all if their prices drop to zero, minus one, or some imaginary value. They were never meant as an investment or a quick buck.