I've seen someone mention a method using google to search in the craigslist.org domain, or some such. I don't do that, but I haven't ever actually wanted to search nationwide.
Edit: here you go (pared down to just the 25d) - Stick this into google...
"cape dory 25d" site:craigslist.org
Finds the one in Maine, adds one in Tampa (twice - but expired - might be one you missed) and something that's "like" one in San Diego.
I just get a search going in one craigslist, then copy and paste other surrounding craigslists in (leaving the search part of the URL the same, just substituting "maine" or "nh" or "burlington" or "albany" for "westernmass"). This saves a bit of time over typing the search in at each craigslist I bother to search.
With a search like you are doing for a specific item over a large region, I'd set up a bunch of bookmarks, and figure out an actual use for the RSS function (finally.) Most likely (depends on browser setup) you can simply click on the "RSS" box/link in the lower right after you search on "cape dory" in "boats" on "maine.craigslist.org" and that will give you a page that will update as soon as any new cape dorys are listed on maine craigslist.
[url]feed://maine.craigslist.org/search/boa?query=cape+dory&format=rss[/url]
Bookmark that, swap in nh, bookmark that, swap in boston, bookmark that, work your way down the coast, and have a series of bookmarks to look through while drinking your morning beverage of choice.
[url]feed://nh.craigslist.org/search/boa?query=cape+dory&format=rss[/url]
[url]feed://boston.craigslist.org/search/boa?query=cape+dory&format=rss[/url]
[url]feed://capecod.craigslist.org/search/boa?query=cape+dory&format=rss[/url]
27 that sounds quite nice in cape cod...get your 2-foot-itis out of the way up front

[url]feed://providence.craigslist.org/search/boa?query=cape+dory&format=rss[/url]
...keep going to the keys, and beyond if you like. There will probably be boat bargains in the gulf this year, for obvious (and unpleasant) reasons.
[url]feed://keys.craigslist.org/search/boa?query=cape+dory&format=rss[/url]
[url]feed://mobile.craigslist.org/search/boa?query=cape+dory&format=rss[/url]
Normally I'm more of a haphazard browser, and very sensitive to distance travelled. Win some, lose some (missed a great air compressor 5 miles down the road last year - got a CD14 project for free this year.) When I was looking for a marine diesel generator, there were none inland, so I shifted my search to the coast and got one a little bit away from my parent's house after a bit of patient (and less haphazard) searching.
Cats, Coffee, Chocolate - Vices to live by.