Honestly, i dont know it is a little risky, but it wont cost that much to get an extended battery and/back cover from amazon.
I have an extended battery for my GNex, and it was like 15 bucks, on sale of course, 20 orig, and works great.
It all depends on the condition of the phone, i would say it is going to have to be a play by ear thing, where you will have to try it out for sometime.
The only problem with reconditioned phones are, they either had a problem, were sent back and fixed, or someone just returned it, so you either get a phone that had a prior problem or possibly a phone that didn't.
It is all luck of the draw man.