Thanks for that. Interesting, indeed.
I have a theory that the whole thing is tied to the level of the item being offered, and not the character level. Essentially, yes, vendor stock is near to your level. But not entirely. At level 2 you may see items of 1 - maybe 3 levels. At level 200 you can find 190-210, let's say.
A level 1 item will need a certain amount of points into bargaining to become yellow 100% of the time, but a level 3 item would need a percentage higher. Since the skill level requirement for that range is so low, the requirement doesn't grow that much.
But a 190 item vs a 210 item would probably require a higher % increase than 1 vs 3, and since we're dealing with bigger numbers here, the requirement will noticeably go up. This is just a theory, though.
Heh, essentially it does mean keeping the bargaining skill relative to your character level, but this may explain why double the points for a level 2 character does not yield the same result for a 100 level character.