In Diablo 2, there are no gold sellers. Gold was completely rejected as currency because it was literally worthless. WoW gold is not worthless, in fact a quick perusal through google reveals that the going rate for WoW gold is about 1.5 euros/thousand (£1.30 or US$2.10). Try searching for gold sellers that actually sell gold for Diablo 2 - you won't find any because there isn't any demand for it.
THAT is the value of gold in those two games and that is how you know one has a better balance of seed vs sink over the other.
Now, some players do have tens, even hundreds of thousands of gold stashed away. However, this is an example of individual player activity and not the overall health of the currency. Blizzard obviously know how much gold people tend to stockpile and are mindful of that when adjusting the seed:sink ratio. This is actually an important factor to consider as Blizzard (and every other gaming company that runs an MMO) will never default on the ownership of in-game currency. Result? No matter what happens, if the player cancels that account and never returns, all that money is gone from the system.