I tried to search but didn't see this anywhere. Apologies if I missed it.
Infamously, the Thurgadin bank NPCs (Mort and Gage) would thank the player for every item handed to them. This was not restricted to bank NPCs, though. On p99, the NPC says nothing when given an item (except for a quest npc with quest-related items).
Evidence:
Mort and Gage thanking for raid items, Sept 4 2001
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Mort and Gage thanking for raid items, Aug 29 2001
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Mort and Gage again, Aug 26 2001
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A young halfling thanking for receiving flurry loot, Aug 18 2001
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