Careful what you whish for. That sounds exactly as one of those fuzzy algorithms that will hide posts randomly due to stupid reasons. Eg. some address has a glitched road and Google Maps believes it is only reachable by boat or helicopter.
This is craigslist, not facebook: don't over-engineer what you don't need to over-engineer. You just look at already-made datasets that give the real world distance between two places, and remove search results based on that.
Never, ever, use google maps for this, and definitely don't use it "per query, as needed". The overwhelming majority of infrastructure doesn't change on even a yearly basis: use someone else's precompiled dataset, and update whenever they push an update.
(aka if you actually need this for your project, you already know how to search for the things you need. If Craigslist needs one-time access to a GIS system to build an initial dataset for high volume countries, they can easily afford the limited time access and dev time to generate the dataset they need, as well as afford the dev time required to do a deep dive into which -free or paid- datasets already exists and are suitable, or not, for this purpose, as well as what licenses they come with)
