When you want to buy something off Craigslist, and the buyer never responds to you. This has happened to me three times in a row now! I see an ad for something I want, I email the seller to say "hey, is this still available? I'm interested in buying it!" and then I never hear back. I mean, c'mon people! I'm offering to give you money in exchange for something you don't want or need anymore! If you've already sold it, then take down the ad and tell people that. Geesh!