To me at least it's usually pretty easy to sift the grain from the chaff in the email responses. Spam/scam messages almost always contain at least one of the following "red flags"
-No mention of the actual thing your selling, lots of generic terms "is your item still for sale?" ect.
-Some long winded "explanation" that would seem like the person put a lot of time into their response and is thus genuine, but if you look carefully you'll see it could be applied to any number of things for sale. If the email is more than 2 or 3 sentences, chances are it's ********.
- Any offer for western union, wire transfer. Steer clear. If you live by "cash and carry" you'll have no issues here.
-Someone requesting shipping, especially in a case where the expense of shipping the item is more than it's worth. Somehow I doubt there's a guy in Africa that wants to pay to get my $300 junker quad shipped to him.
- Excessive spelling/grammar mistakes. Not everyone types like an English professor, but ones that look like they were written by a 4 year old are usually BS too.
People do this crap to get you to reply(DON'T, a response telling them to **** off still gives them your "known active" email). They do this a few thousand times, and harvest these emails. Marketing companies pay good money for lists of known "good" emails so they can send spam mail to.
It's also why you should not put your phone # in ads. There are people that do the same thing, harvesting #'s out of ads and selling them to ad agencies.