My problem with buying used boots is that it's so difficult to find the right boots even when you're in the store with a ton of selection. If you're going the used route, I would think you'd have to drive around a lot before you found the right ones. Would you end up saving any money?
I feel bad for the renters. They have to wear old, nasty, used boots that have probably seen many feet in their lifetime. I only rented once and that was for a short weekend. Luckily for me I have big feet (size 13) and the boots they gave me have been hardly used. My buddy on the other (who was also renting) had smaller feet and his boots looked real shitty.