Those are exceptions though - often because there's something rare about it.
The LSL collection isn't rare - what IS rare is the Quest for Glory Anthology which will always fetch over ?30 on ebay.
Other rare games:
Touché The Fifth Musketeer -- sells for ?50-100 easily (I know, I sold one copy for ?100 once
)
Discworld II in original packaging (none of the re-release stuff)
For a good idea, check Gametz and check out the "wanted"/"available" ratio.