The second hand market for PC gaming is very poor...
Steam games once installed and assigned to a Steam account can't be resold. Most new UBISoft titles can't be sold on...
Most older titles fetch very little, its always been IMO that once you buy a PC game, you've pretty much got it until you throw the CD or DVD or whatever in the bin...
Steam games are the worse.. Not much chance of shifting them off.... Which is troublesome if you sell your gaming rig.. Left with a lot of games you can't even pass on to others, unless you give a single lucky person your Steam ID 0 and you can't even sell it, against Valves regulations on steam accounts..