Why the difference in resale value?

The Adcom GFA-555 and the Onkyo Integra M-504.  Two relatively close-in-price power amplifiers that emerged in the mid 80's to critical acclaim from the audio press. The asking price for the used Onkyo on sites like ebay is frequently higher than its original MSRP; for the Adcom, not nearly as often.  Why is that?