Correct the balls are marked, if you watched, every time Bonds came up to bat (Or Sosa, or McGuire) the ball was changed by the ump.
And as was earlier pointed out, I doubt McFarlan was looking to make a profit, since he also bought several of the later 6x balls.
All this reminds me, need to DL my picture of #73 getting hit. I was at the game, sitting in the bleachers…alas, wrong side, we sat behind Bonds.