Usually used parts have a 30 day exchange only gaurantee. The starter you have in hand was rebuilt, right? Consider this idea - get the junkyard starter and put it on. My bet is it works fine for a long time. Keep your broken one, the only thing wrong with it is the mounting flange.
If you still have the car when the junkyard starter quits, you can transfer the mounting flange from the junkyard starter to the rest of your rebuilt one.
Keep in mind that Autozone will almost certainly charge you a core charge and won't accept your broken flanged starter as a core. You might as well ask, but usually a broken as opposed to worn part won't be accepted as a core.
You never said if the car is a stick or auto. If it's a stick and you live in reasonably open country with some hills such that push starting is not that hard, definitely go with the junkyard dog.
If it's an automatic, and/or you have to work outside with no garage, and/or you really have to depend on this car, you may be better off paying more for a rebuilt one.
A point, though - if you have a "real" import auto store, see if you can get a Bosch or Nippondenso, new or rebuilt. Some of the rebuilt parts from the big chain stores are not up to Bosch or Nippondenso standards.
The junkyard starter may indeed be as good or better.
Neither one will compensate you for labor, if you are doing the swap or if a shop does it.
I'll leave you with 2 of my favorite quotes from Clint:
"A man's got to know his limitations."
"Do you feel lucky, punk?"
In any case, don't forget that you need one more bolt, the one that Mr. Bodger left off.
I'm betting you put it back on in way less than the time it took you to worry it off.
If you are working with a very limited selection of tools, consider getting a more extensive set, maybe Craftsman - they are still pretty good.
Your McGuyver stunt tells me you have better mechanical sense than a lot of "pro" mechanics out there - the ones that are not really car guys, just sad sacks that needed a job and fell into wrenching.