Used to buy a lot of ex currys demo/ex display/slightly used dysons from an auction house in the midlands, that is until Dyson realised that they were probably losing sales and changed the system so that all machines went back to them direct! I also see that the machines on display in currys now have big property of Dyson stickers on them... Nowt like keeping it all in house hey!
Friend of mine works at macro and tells me when they have dyson promos involving part ex's on that they are ordered to make sure all part exs are sent back to Dyson for destruction!
I guess by keeping such tight reigns on used stock they ensure the new models sell well.