Finally figured out what was happening:
On Saturday, when I first saw the CL listing, I clicked the CL seller email address to send a message to the seller, and Windows Mail opened. I don't normally use WM. WM errored, i.e. the message never went to the seller. Having seen that there was an error, I resent the message using my usual email client. However, I didn't close WM. WM's default is to download email every 30 minutes and delete the messages from the server. So, I never saw the seller's response from Saturday. Just today I noticed that I had ~20 messages in WM Inbox. I tried to forward from there but still got errors and I called my provider (ATT/SBC/Yahoo DSL). After around a 30-45 minute call, the CSR was able to finally determine the settings necessary in WM to enable sending messages. It was fine downloading, but sending required a different batch of settings than those used by my Forte Agent email program. Mystery solved!