Craig's List has cracked down on that. If you ever list anything for sale on there, you'll find that the people who respond no longer "reveal" their e-mail addresses. Instead, when you e-mail
CraigsAd394302@cragislist.org to ask if the big fish tank is still available, the seller gets an e-mail from
RandomStuff@craigslist.org, with your real address hidden.