On my personal income, I’ve been working with Craigslist for the past 2 years and here are my comments
Know how Craigslist works. Whoever emails/calls first wins. If it’s free, you need to pick up the item almost immediately. Good free items are spoken for within seconds. Depending on the city, there are hundreds of people sitting around trying to snag free items. Same for cars, electronics, computers, and other highly competitive sections. Good pictures are extremely important, as are clear descriptions. General sections are refreshed just under 10 minutes, sub-sections about every 15 minutes.
Value your time. Figure out how much you want to make each day and break your day into sections. Average profit for items I pick up ends up being $60-$70. I aim to pickup 3 items a day. I’m not worried how many I sell each day because everything eventually sells. When I arrive at someone’s house, I’ll ask if they are selling anything else or giving anything else away in case I can boost my profit per trip.
Selling advice. There will always be someone that will buy your item. It might take another hour. It might take another day. It will sell. If you are getting calls on your item then you probably have it priced right. Remember that you don’t need to sell to the first caller. Or the second. What you need is to make the most money possible on your item.
Buying advice. Ask lots of questions on the phone and be very specific. There’s nothing I hate more than showing up at someone’s place only to find out there is some deal-breaking problem I wasn’t told about. Negotiate over the phone/email or plan on paying full price. When I’m not paying close attention, I make mistakes. Profit is in the details.