Good idea but it might be difficult to implement. In a country like India, custom clothes are quite common due to the availability of cheap labour. But the most crutial part of that process is the measurements and the actual cutting of the cloth. Though the cutting can be automated, the measurements are almost an art form and usually done only by the master tailor.

So you might end up with a lot of returns if the customers mess up the measurements (and remember, to properly measure pants, you need another person doing the measuring). You could take measurements off and existing pair, but that may not be as accurate.

they already have this through cafepress.com except you can't custom tailor the size and the clothes are sometimes cheap. There's 2 different projects similar to this right now. Nike has contracted Yves Behar to make a learning shoe that tracks the contours of your foot. You wear it for a week and a week later you take it in and nike makes a shoe on the spot for you. Additionally google's ceos just bought another internet business that does just what you asked. The store is called Zazzle