My elderly mom and dad have started conducting Estate Sales for people (a garage sale where you are selling off everything in the whole house), with a great amount of success. They are semi-retired but wanted to bring in more income. They don't get paid up-front but rather take a 40% cut of the profits from the sale. On average, the sales generate anywhere from $4k to $10k.
Usually what will happen is an elderly person dies. Their existing family then wants to sell the house. Most of the time, the family will hold on to a few keepsakes, but has no use for all the hundreds of other items in the house. This is when my parents are contacted (either by neighbors, friends, real estate agents who know they do this), and they will take it upon themselves to sell off EVERYTHING in the house (from couches to spoons to cereal boxes). They post ads on craigslist & local newspapers announcing the weekend sale, then price out everything in the house, then usually have me or someone younger help them with the heavy lifting (beds, dressers, etc.) that Friday and Saturday. (As I said, they are elderly).
The families that they do these sales for are always so surprised by how much money this seemingly worthless stuff can generate. I think the expectation to make money is so low (often families will just donate all the contents of these houses to Goodwill) that everyone they have conducted these sales for has given them rave reviews.
It does help that a) they live in a middle class neighborhood where people generally have more valuable items in their homes and families are more inclined to want someone else to do their dirty work instead of conduct the sales themselves. AND b) my mom has a good knowledge of antiques and jewelry. She's caught a couple $500 items (ugly frog figurines) that could have easily been sold off for $5 if you didn't have a discerning eye.
The cons: people steal A LOT from garage sales. You need to watch over them like hawks. Definitely need 3 people in the house at all times. And, you have to sift through some pretty gross stuff sometimes.