Buying a used tire to carry as a spare?

I always use the best of my take-offs for my unmounted spare that I carry. Since I change tires at around 50,000 miles(3-4 years) they usually have tread left and I know the history of that tire. I also get a decent credit for my tires when I buy new tires. Changing a tire is not that difficult and torquing the lug bolts or loosing the lug bolts is not that difficult either, with a breaker bar and a pipe extension, held with a jack stand. In fact, if you want to carry a couple of tire irons, you can mount the tire just like a service tech will. The tire bead can be broken with one of your levelers. It all depends on your physical and your mechanical abilities.
If you don't mind waiting and being at the mercy of the nearest tire dealer, that is OK also. To each his own as there is no right way or wrong way when it comes to a spare tire.