This is a "slippery slope". As both an occasional buyer and seller on Craigslist, I know there is a delay at times during the communication phase.If you’re selling maybe check your messages!!!
Fortunately, I have a job that allows me to look at my phone when it alerts me that a message has come in...any time of the day. Not everyone is that lucky. Some folks have jobs where they could be fired if they're caught on their phones---some have jobs where they have to leave their phones in their locker (etc...) and can MAYBE look at it during their lunch break; or even wait until their work day is over.
So; not everyone can answer their messages right away, and some of them may not even know they have a message waiting until they get a chance to look at their phone.
Now; if it takes more than 24 hours for someone to answer their messages, I see no reason for. ESPECIALLY since everywhere you look everyone's got their face glued to their phone screen during their free time.
As far as the "email" link (messaging system) goes; I warn all perspective buyers that I will automatically delete them without opening them. I give my phone number and first name, and ask for text or phone call ONLY...every other method will be ignored.
So...if there is a DELAY in messaging there could be a good reason for it. If they NEVER get back to you...they are an ***hole.
This brings me to MY rule:
1). Once the item is sold TAKE THE AD DOWN!!!