It's generally pretty easy to weed out the time-wasters and scammers on Craigslist. Simply ignoring any emails asking 'Is it still available?' is the best advice I can give. There is no obligation to interact with anyone you don't want to.
In roughly 6-7 years and about 80 sales, I've only had maybe two people pull me into a back-and-forth that tuned into a frustrating waste of time.
Online selling (and buying) is what you make of it. I can be and should be easy.
I always put "if this ad is up, it is still available" in the listing. Have to weed through a few morons that I just hit delete on, but someone will always come along and send a coherent email and be super easy to deal with. Not every message requires a reply, you can tell from the first one if they're worth your time or not.