why is a used hp 5000 worth more than a used hp 8000 / 8100?

Could someone explain to me why a used hp 5000 is worth more than a
used hp 8000 / 8100 (on ebay etc.)? no matter what the page count hp
5000s seem to go for more than 8000 / 8100s even though the 8000 /
8100s often have duplexers and the 5000 duplexer is usually flakey.
(the 8000 is nominally 24 ppm and the 5000 is nominally 16 ppm and
both are 1200 dpi resolution IIRC.) Thanks.

-Confused