Pant sizes are all over the place. Some stretched, some have shrunk, so you can't go by pant size. If you research this more on this site, you will find all kinds of stories about mistakes and lessons learned about using pant sizes. Also depends on where they wear the pants, under or over their dunlap tire!
So for Etsy or other places, you would likely be better off making standard sizes to stock and sell.
If custom making, you need to set up a measuring strap as other threads have suggested, so you get a real measure you can more likely trust, as long as they are honest in measuring and recording the number.
Tom