This guy is selling off a large collection pretty cheaply by UK standards - link to his ad is below:
Unusually for Gumtree, he appears to be ready to do mail order (at least in the UK) as well as allowing people to collect them from his home. While I didn't actually count his disks, he's definitely got several hundred - I saw them - and all of the ones I bought are in reasonable used quality, ranging from ex-rental to very good.
I don't know how long this will last - he's apparently moving soon and needs to shift them before he goes. One warning - he's on the fourth floor (US fifth floor!) of a block of flats with no lift, getting large quantities downstairs would not be fun!