I like this idea, although it seems really hard to add that to the game. If it links to the actual vendor, what happens when the vendor falls? I've seen rented vendors at my house fall for absolutely no reason many times. The vendor had hundreds of millions on it so didn't run out of gold, the rent is free so that can't be it, and neither of us canceled the contract... yet somehow it fell anyway. Would all the vendor coupons just be useless because the vendor fell accidentally once?
If it's linked to the character, what happens when the character gets deleted or transferred off the shard?
If it's linked to the account, what about when the account is given away or sold, how would they prevent the old account owner from giving himself lots of 100% off coupons, then going and taking everything off the new owners vendor for free months later?
It just seems like there are so many issues that could come up trying to implement this, compared to just adding the person on ICQ and getting them to lower the price for you in person.