Is bartering/work trade legal if it's off the books?

So I worked for a woman for only a couple of days (she said she was training me) but she followed up and wanted to do a work-trade program with me. For the two days I worked she didn't pay me money, but instead told me to pick out anything from her store. Another girl said she did this with her, but for 3 months. She doesn't file this bartering in her taxes, she just writes down how much you worked on a piece of paper, and subtracts what you get from the store. I just wanted to know if this is legal and can I do something about it.