Webuyanycar sell cars onto registered car dealers, either directly or through 'closed' auctions (where the general public aren't allowed).
Firms buying houses 'instantly' offer well below the market value (in the hope that the vendor is desperate for a quick sale) and then place the houses back on the open market, often by auction.
Firms dealing in mobile phones send older models (bought for a pound or so) for recycling, so that they get paid for the precious metals within them. Other models are probably offered through eBay shops.
Those buying CDs (typically for around 20p or 30p) often sell them on, under the 'Replay' label, through High Street retailers such as . . .
http://www.poundland.co.uk/product-range/music-books-film/cd/