Herb's Stuff (New Jersey, USA) looks quite promising. The web site has been updated recently. "Machines are sold as 'working when shipped' unless noted otherwise."
eBay usually has some available. I've not used eBay myself yet.
craigslist has free online classified ads, with a section for computers for sale. There are seperate listings for each city where it is available, which is nice because shipping can easily cost more than an old Macintosh. But only a very few ads deal with old Macintosh computers. I obtained a Macintosh IIcx through craigslist. (Thank you, Lil and Sherm Sundet!)
applefritter Marketplace has two forums, one for the "sale and trade of Apple-related computer equipment", and the other for "listing of Apple-related computer equipment you are looking to purchase". Only a few of the posts deal with old computers.
68k Macintosh Liberation Army has a "Trading Post" forum.
Old Computer Market is "The place to sell, buy or swap vintage, classic or old computer systems and related items." Only a few of the ads deal with Macintosh computers, but they're easy to find since the site is organized so there is a seperate page for "Vintage Hardware > Apple". Thanks to Stefan for pointing out the new address.
LEM Swap List is the Low End Mac email list for "buying and selling Macs and Mac-related goods." An archive of the list is also available (mentioned near the bottom of the page). Thanks to Dan Knight for pointing out this page. It looks quite active, though only a few of the posts deal with 68k Macs (11/21/2005).
Forevermac.com is "Your Refurbished Apple Macintosh Computer Supplier for the United Kingdom". This link is to their "Compact Macs" page.