I thought about selling my things at craft fairs, but no one wants to pay you the money you've got into it. I wanted to work at home, so I started advertising by mail order. When I get an order, I keep the person's name on my mailing list for at least a year. I've published three books on how to use up leftover yarn. That's how I got started.
Some people think leftover yarn is only about an ounce or two, but it can be several pounds.
It varies, especially if you buy a different brand than the pattern calls for. Then you need something to do with it. I make a large candy cane decoration for Christmas, and with a knitted bow on top, and an Uncle Sam caricature with the knitted legs and body in one piece, and the hat and feet separate. I have a few patterns for kleenex and toiletpaper holders using cotton yarn, and people really like my baby sweater made with one ball of yarn, for someone on a supertight budget. I've lived in seven states, so I have a lot of experience with what people want.