This one might take a playthrough to unlock higher quality stuff. Style is based off of two specific factors: The quality of the racks/vendors placed in your store, and the quality of the items placed on the racks themselves.
In other words, buying a bunch of Evil racks and placing some Demon/Angel gear (8th/9th dungeon drops) on them will more than suffice for this achievement. Note also that the racks are worth the same amount of points regardless of size, as well as gear of the same tier as each other. In other words, placing a ton of 1x1 high tier racks and placing high tier jewelry on them will give you FAR more style points than placing 3x2 racks with armour on.
For perspective: Evil Rack is worth 18 style each. Demon quality equipment is worth 12 style each. Each 1x1 Evil Rack with a Demon Pendant on it would be 30 style. :3