I'd be really concerned about your roof decking and rafters if dropping a few bundles of shingles on the roof caused your ceiling drywall to crack. The vibration from that shouldn't affect drywall seams like that at all. Nor would it have a thing to do with the roofer. It has more to do with the house's construction possibly being inadequate. You need a home inspector or contractor to take a preemptive look from inside the attic to see what they can assess. You don't want to get dinged on that from a buyer's inspection.

The drywall didn't crack - it let go at the seams. It's an older house from the 50's - a lot of 2X3 interior construction - not sure about the framing- typical in this area. Feeling the house actually shake does not surprise me to see the damage on the low slanted ceilings...