How is buying the entire market good for the beastmen? The market's goal is to sell beastmen. The entire market being sold all at once, at full price, is a windfall for them. As long as the trade of beastmen is legal, they just need to go and "procure" more from wherever it is they kidnapped them from in the first place. She just rewarded the dude that keeps this infamous place running. And unless she has some plan for the newly bought beastmen, they could just be caught again.
Not to mention the detail that she was told she only had "a little" money available since she was only buying using her father's name. Even adjusting for being the richest family in the empire, which means that "a little" for her would be a hefty amount, can she just buy a whole marketplace? I suppose the father may overlook it since she did put down that guy who humiliated her before. Nothing more satisfying than slapping a dude with a wad of bills to teach them their place.