The people who are selling the same exact 10 pins over and over buy them from counterfeiters in China. They pay pennies per pin, then sort them into lots and sell them on eBay. That's why we don't buy pins on eBay anymore.
The "sharks" are the ones who will trade you one of their pins for 3 or 4 brand new rack pins, then they sell the new rack pins on eBay. While those will sell for less than full price, those are not the ones you see with the same identical pins, in the same identical lot, under multiple user names, day after day for months. Nobody just coincidentally has a hundred each of the same 10 pin designs.