In some senses you can compare the second hand games market to fractional reserve banking.
Everyone in the chain resells the game after they have had their 20hours of use... user 1 sells at 90% of value, then user 2 at 80% of value and so on... As others have pointed out this cycle allows a reasonable proportion of new titles to be sold at the £40 bracket as users trade in 2 fairly new titles in exchange for a 'newest' one.
A bit ike a bank taking in 100K of deposits and using that to lend you 100k , then the next person 90k and then the next 80K and so on as would be the case with a 10% reserve requirement. Ultimately this props up a house price bubble as banks lend more than they have, and NEED to keep lending to keep the cycle moving... much like the new game price bubble. (I know high end console games have always been £40ish since the mid 80's... so they havent risen to subsequently fall like a true bubble... but surely games are due the kind of 'price realignment' suffered by CD's DVD's, ebooks etc)
Unfortunately software houses are now trying to chase these incremental price points and dropping the software price almost daily. Look at the retail cost of the new tomb raider game... literally halved in 4 weeks from the date of release and its been on a slippery slope since. What confidence does the initial buyer have that they are paying a 'fair' price. What confidence does a softwarehouse have that they can recoup their investment or be rewarded for their hard work in producing a decent and fairly ambitious game?
If you want a window into the prices Sony and M$ think they can charge have a look at the PS store. £60 for a new (download) title, when i bought a second hand physical copy of far cry3 for £12 they were still selling far cry 3 download for £60.
Interesting to see what comes of the steam console.