I use Google Alerts to set up a search string and each time that comes up, for instance "450SLC 5.0", it sends me an email advising me of the new find.
I used this method to locate and purchase both of my 450SLC 5.0 Mercedes-Benzes.
You can also use something like this in the Google Search box:
"Mercedes 450 SLC 5.0" site:*.craigslist.org
This will search for the string "Mercedes 450 SLC 5.0" in any (that's where the * comes in) craigslist site nationwide. If you refresh the search or use it too many times in a row, Google thinks you are some kind of data-gathering routine and cuts you off, though.
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