Plantain is considered a nuisance weed in Alberta, and if there's something growing in your lawn that isn't a dandelion, it's probably a plantain. And while the vendor was probably telling her customers about all the plant's reputed herbal uses, she probably neglected to tell them how it spreads aggressively by seed and is difficult to eradicate.
Major Plantain, photo by Sri Mesh
On the plus side, young plantain leaves are considered edible and are sometimes used as a wound dressing. The plants will also usually grow without supplemental water or fertilizer. Unfortunately, they are not very attractive plants, and anyone who grows them will likely find themselves with a lawnful of plantain seedlings and some unhappy neighbours.
However, if you're out to make a few bucks, I suppose a good way to do it would be to grow plants that are easy to grow, from seeds you collect in a vacant lot, and sell them at high prices to unsuspecting, beginning gardeners at a local farmer's market. The moral - don't buy plants on a whim unless you know exactly what you are getting.