I would disagree that someone who, for example wants to avoid being contacted by maybe many many scammers on the site, "likely" holds an attitude.
I mean, I'd prefer they put that message up than, say, something like "Deposits only- nonnegotiable"
I also don't see how the statement conveys lack of knowledge about basic customer service. Saying out front that "people who aren't serious should not contact" does not at all betray ignorance.