As to your question on why you can just give items away... each npc has a attitude rating toward each of you companions that can be raised by gifting gold or goods. You can see this by right clicking and examining. After 200 gold or equivalence in goods the attitude will max out at 100 and that npc will offer that toon better buy and sell prices.
This is per npc and per companion. If your main donates 200 gold to a npc that npc will only give your main the discount.
Honestly, that just seems incredibly silly. Discounts for doing specific things for merchants (or saving them from such-and-such threat)? Sure. Discounts for... giving them random stuff? What? That makes no sense.
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