If you're not a frequent poster on Craigslist, the algorithm there probably doesn't trust you enough yet to let you post on a category for a faraway place. You might try posting a few times in categories for your local area to gain the algorithm's trust before posting again in the category for the non-local place.
Even if you could reach their support people, I doubt they would be very forthcoming about countermeasures to their anti-spam provisions.
It's also possible that the IP address your ISP assigned to you was once assigned to someone local to you who did bad things on Craigslist and it may have been blacklisted. I doubt they would tell you that, either.
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