Some recent serious games auctions on eBay:

Serious Sam (GameBoy Advance) USED (cartridge)

$15.00
End Date: Thursday Feb-8-2018 11:11:10 PST
Buy It Now for only: $15.00
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Foodie Fight: A Trivia Game for Serious Food Lovers - New Factory Sealed
$13.99
End Date: Tuesday Jan-30-2018 15:26:26 PST
Buy It Now for only: $13.99
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Serious Sam Collection (Microsoft Xbox 360, 2013) No Manual
$11.99
End Date: Tuesday Feb-13-2018 18:02:13 PST
Buy It Now for only: $11.99
Buy It Now | Add to watch list