Mount the 10.5 tires you already have on your new rims. You will be amazed at how much better and wider they will look on the new wheels with the better backspacing. If you bought the BFG's new, you will not get what you have into them by selling them used, even if only a little used. When they wear out, then get you 12.5's if you want them.
That is what I would do.
Edit: added a pic for you. I know it is a Cherokee, but my buddy has 31x10.5x15 tires on it. Look at the difference the aftermarket wheels made with the same tire going from stock wheels to an aftermarket 15x8 wheel!