I am SOOO frustrated with the people on Craigslist!
I don't know what to tell you about the dog, I sure hope you hear from them, but honestly, I wouldn't hold my breath.
I have answered many ads on there wanting something they posted, and I have posted several things, and the result has been a consistent frustration for me.
The ads I answer, I get very little response from, and the adds I post, people email me about whatever I posted, set up appointments to see it, and NEVER show up! I wait around for hours for these people, it is SO RUDE!
I guess it depends on how bad they want to get rid of the lab....Is there a phone number in the ad you could call? If not, I would email them again, and ask if they could at least let you know if the dog is still available.
Hope it works out for you!