I never sell old discs. If I'm done with them or have doubledipped, I'd just give the old disk to a friend and make him/her happy with it. Not worth the hassle of selling it...
I was there, the big BNB blackout of november, 2008. We lost many that day...
For £5-6+ it's worth it but then do you upgrade many DVDs?
99p as much worth as it gets a lot more people interested but it has it's big risks like Romero says. It's a lot down to luck. Remember you can always get a friend to bump it if with things aren't going well.
I'd start it at £4.99-8.99. It's well known Criterion's aren't like any other DVD.
I don't upgrade many DVDs if I can help it but the new 7S has a new digital transfer, new translation, all the extras from the current release plus loads of other stuff.
People won't think £5 is too much