I actually worked on a site that did this way back in 1999.
It's hard because people get fatter and skinnier all the time (usually just fatter) thus, the clothing "refill" doesn't work. Especially for pants. In fact, most men only bought new pants because of a size change, not because their old ones wore out. It works for socks because people's feet don't change.
Another problem is that even for men, standard sizes aren't standard at all. A size 32 waist pants from Old Navy is about a size 38 in Levi's 501. Athletic cut dress shirts are much larger than slim cut dress shirts in the exact same size. For women, you can multiply this situation by the number of retail stores and brands in the world. Sizes even change in the same line from season to season. Finally, people lie about their waist size, even to their computer.
It's hard because people get fatter and skinnier all the time (usually just fatter) thus, the clothing "refill" doesn't work. Especially for pants. In fact, most men only bought new pants because of a size change, not because their old ones wore out. It works for socks because people's feet don't change.
Another problem is that even for men, standard sizes aren't standard at all. A size 32 waist pants from Old Navy is about a size 38 in Levi's 501. Athletic cut dress shirts are much larger than slim cut dress shirts in the exact same size. For women, you can multiply this situation by the number of retail stores and brands in the world. Sizes even change in the same line from season to season. Finally, people lie about their waist size, even to their computer.