Several months ago, Marie* read a story in The Penny Hoarder about BookScouter. She had been selling books online for a while, buying used books at garage sales and library book sales.

Until she read about BookScouter, she didn’t realize that Amazon wasn’t the only company buying used books. She learned quickly that other companies often offered more money than Amazon did. BookScouter routinely lists prices from 30 or more buyback vendors.

At library and garage sales, she now uses the BookScouter mobile app to determine if a book is worth buying. Because she lives in a college town, students have become a reliable source of textbooks. They come, they stay, they go, and they tell their friends about her.

She sells books on both BookScouter and Amazon, “depending on how fast I want the money,” she says.

Buying and selling books is a “casual” operation for Marie, she says. She uses whatever money she makes–last year, it was $1,600–to enrich the family’s vacation: instead of driving to a destination, they might fly. On a recent trip to Florida, her book-selling money paid for surfing lessons for the whole family.

Marie has alerts on her computer telling her when students post books for sale on social media sites, and she keeps cash handy to make quick, convenient transactions. Students in her town wanting to sell their books contact her directly, she says, and usually get cash that same day. Her goal is to make 25 percent over her investment.

She is fair with her customers — if they’re selling a book for too little, she will sometimes pay more than what they’re asking. If someone’s asking what she thinks is too much for a book, say $200, “I won’t even make an offer because it’s way too high,” she says. “But it all works out.”

*At Marie’s request, we’re not using her full name or revealing where she lives.