User blog comment:GnarlyToaster/Ohi guise, guess what?/@comment-1808057-20100801203602

If 21 items are sold, the 1st one to be sold is removed from the buyback list. So if the COM was sold first before a bunch of loot, then...it's gone. But no, it doesn't refresh with the machine. It resets when you leave the zone (Crawmerax's Lair), however.