The REAL problem
Why are people trading in their single player games so early to create this market in the first place.
I still have the majority of my SNES and Megadrive games, why? because they're timeless. I can play them over and over again, they have lasting gameplay value on reliable systems which have outlived numerous failing 360s, PS3s, and Dreamcasts.
360 and PS3 games with the same long-term appeal are rare, I have one or two which I'd never consider trading in, but that's it.
The problem with most AAA games if that they don't have this appeal, they're shallow games propped up by a bunch of pointless extra achievements and downloadable content in an attempt to keep you playing. Once I've played them through they get traded because I'd have no real desire to play through them again, and probably won't have a working 360 in a few years from now once they're out of production and all failed; look how quickly Microsoft dropped the original XBOX. Furthermore I don't really consider them worth the price I paid for them and the trade in money subsidizes that. If the trade-in option wasn't available I wouldn't have paid the 'new' price.
Current games are a short term fix, like a drug, not long term investments, and this is even more true of the downloadable content.. Due to this, I don't value them as long term investments, and trade them in.
Braben should be looking at the cause of the problem, not trying to treat the symptoms with an axe. I find his comments ironic when you consider that Elite and Frontier are two of the most timeless games around.