Usually, these legendary telepaths would ask for a few mind fragments in exchange for the food they sold. As long as they got more mind fragments than what it cost them to make the food, they would profit.
It was a classic case of profit = revenue - cost. Legion was hoping to take advantage of this business practice to get a lot of the bait that they needed without troubling themselves too much.
The problem was that the legendary telepaths only sold a few memory fragments as food to their customers, and they would get suspicious when a customer was asking for a large amount of the food without eating it.
Legion could have pretended that they were eating the food while secretly accumulating what they needed for bait over a period of weeks and even months. But they didn't want to waste time on acquiring food when they still had to waste a lot of time waiting for prey to take the bait.
