Do you really need this explained to you?
Let’s say there is ten million gold total in the economy. Yes, I know that is an exceedingly small number, but work with me here. I myself have three million of that amount. I give a million each to three different players. How much gold is in the economy now? The amount is STILL ten million. I myself have three million less, but overall nothing was lost.
That is NOT a gold sink. In any instance where gold goes from player to player either via a direct trade or the auction house (which DOES take a 5% cut as a gold sink), then you are only moving the gold from player to player. That gold STILL exists within the economy. A gold sink REMOVES the overall amount of gold from the economy. When you purchase an item from a vendor, pay for gear repairs, unlock bank tabs, or transmog your gear, the gold does NOT go to a player. The game itself removes it from the economy.
Quick example: let’s say they put up a new mount for three million gold. I buy that mount. The amount of overall gold in the economy has now dropped to seven million.
THAT is a gold sink. It isn’t whatever the hell you seem to think it is.