I am just a customer - but buying used balls and having them plugged and re-drilled is a common practice for me. I never had problems with this. Surely, this doesn't generate that much sales like a new ball would do. But my local pro knows what good service is, and that only a satisfied customer will 1) come back for more and 2) will tell others about his good service.
In my short bowling career, I had already 8 balls redrilled, only 1 new so far. But my pro always supported me with the used stuff, because I told him that I'd like to experiment with different coverstocks and manufacturers before buying a NIB ball which will fit my game well.
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DizzyFugu --- Reporting from Germany
"All that we see or seem, is but a dream within a dream..." - Edgar Allen Poe