Some recent serious games auctions on eBay:

Serious Sam Microsoft Xbox Video Game Complete

$10.94
End Date: Friday Jun-30-2017 18:15:23 PDT
Buy It Now for only: $10.94
Buy It Now | Add to watch list

Serious Sam Collection (Xbox 360) (4 games + DLC)
$14.99
End Date: Thursday Jul-27-2017 8:12:50 PDT
Buy It Now for only: $14.99
Buy It Now | Add to watch list