I always check to see what else a seller has been doing, usually by searching for their phone number in other craigslist ads.
For this seller, the appropriate thing to put in the Google search box is the following:
"817 313 1783" site:craigslist.org
Note that the double quotes are necessary because we are searching for the phrase of the whole phone number.
The site command limits the Google search to only craigslist ads.
This is a good way to search for a seller's past activity, but not for searching for new ads because Google's database of craigslist ads is so old.