When a response to my Craigslist ad shows a major lack of competence or reading comprehension, I tend to think that the interaction would not be worth the trouble, and don't reply.

It is surprising however, how many "spam" responses turn out to be from real people- when replying to a for sale ad myself, I try to demonstrate that I am a real person- where I'm coming from, when I could look, etc., and never ask "is this still available?".