If you are using DHCP (Dynamic Host Connecting Protocol) from your ISP, try to switch off your Modem for a few minutes, perhaps an hour.

There are blacklists for certain IP's and Craigslist might check them.
If another user had that IP before you and did something that didn't go along with the policies, chances are that this IP was automatically blocked by Craig's.

This is just a guess!

Once you switch off you modem for a few minutes, it will have to drop your current IP, therefore it may take as long as an hour. After you boot your modem again, the ISP's DHCP might give you a different IP.

Dunno if this will fix it, but it doesn't hurt.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert