A milliner is someone who designs or makes hats, especially women’s hats. Milliners usually sell hats too.
The noun milliner is a somewhat outdated word for a hat maker — it refers to someone whose business is to design, make, decorate, or sell hats. You might go to a milliner before your trip to South America, to buy a large-brimmed sun hat. The earliest meaning of the word milliner, from the fifteenth century, was "vendor of fancy wares, especially those made in Milan," though by the eighteenth century it meant "one who sells women's hats."