One of the many ways to run Macintosh 680x0 software is to actually own a real Macintosh 680x0. Also, almost any emulator (including Mini vMac) requires a ROM image file to run, and so can be legally used only by those who own such a Macintosh. This page has some ideas for where to buy one. Let me know if you know of any other places.
eBay usually has some available.
68k Macintosh Liberation Army has a "Trading Post" forum.
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).
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.
Marktplaats.nl is described by "Addy" as "the largest second hand site of holland". Searching for old Macintosh model names turns up a few.
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!)
Herb's Stuff (New Jersey, USA) "Machines are sold as 'working when shipped' unless noted otherwise."
Operator Headgap's Mac Resale Store in Memphis, TN, USA. "Custom & Refurbished Mac Systems, Equipment, Parts, Software, & Accessories."
myoldmac.net has a page to help find old Macintosh computers on eBay.
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