Just so we're clear here: flipping knives for a profit without a "Dealer" membership is against forum rules and is wrong to do on this forum.
The best thing we can do is be the example we want the Bladeforums community to be.
I spent some time reading the rules and looking for posts/stickies by the Mods or Spark, and I find no rule against "flipping knives for a profit without a 'Dealer' membership' ". I agree that the practice goes against the spirit of this forum, particularly when engaged in by those who have no other stake here and your last quoted sentence is spot on. I greatly appreciate the sense of community here.
On the other hand, I think that negotiation is just part of the process when buying and selling in an open market such as the Exchange. As your buyer asked for a discount, I would assume that he would expect to be asked for the same consideration by a potential buyer when selling his new/your former knife. Thus his price might have been set so that he could come down to make the sale; cover his shipping, PP fees, and other costs; and maybe break even.
I've come down on sale prices because I wanted to move on and recoup what I could. I had no feeling of philanthropy or generosity in doing so--just wanted to make the sale. I had warm and positive exchanges with the buyers and became friendly with some, but didn't care the least what they might ask for if they decided to sell. There's a number of people here that buy and sell a lot of knives not for profit, but because they like buying and selling and handling a lot of different knives. I don't think most of them make any money at it, they just set their initial price-points to try to minimize their losses.