Construction inspectors have to pass a long code exam to get certified, and the study material is scattered across thick code books. One inspector turned what he knew into an online course on weekends. It made under $5,000 its first year and about $45,000 a month a few years later.

Old concert and wrestling T-shirts sit in thrift-store bins for a few dollars, and the real ones sell to collectors for hundreds. The skill is telling a 1990s original from a reprint across the rack. One reseller has sold more than 10,000 of them on eBay.

Every company wants matching, professional photos of its people, and nobody wants to leave the office to get them. One San Francisco photographer started emailing coworking spaces and shot 1,500 sessions in his first studio year. The studio is the client's own conference room.