lostarchitect 

Just use your head, basically. A lot of spam / scam emails are sent by bots--they don't read your post, so they don't know to include a phone number. Note that you won't respond to emails asking "Do you still have it?" and you won't respond to emails that don't note the specific item for sale--Then ignore emails in these categories. Better yet, report them as spam. You have now eliminated almost all spam and scam emails.

Another note, saying you won't respond to someone who doesn't give you their phone number is probably losing you sales. I don't know you, I don't want you to have my phone number until I'm sure we're going to have a deal. When selling or buying on Craigslist, I don't share my phone number until we have set a time and place for the transaction. At that point I share my cell number in case the person is going to be late or can't find me. I recommend removing your requirement for phone numbers. It doesn't reduce spam but does reduce actual sales contacts.

I additionally note, for the same reasons as above, that I won't respond to emails that ask me to call or text them to talk about the deal. Sorry, we can talk about it over email.

Beyond that, really good, big pictures are key.