As I've mentioned before I first encountered transaction hiding in World of Warcraft. There were 'grey'[1] items in the Auction House with a buy it now price of 500 gold (which at the time was extreme). I could not figure out who would buy a twill vest for 500g until it was explained that you went to a web site, paid them some cash for gold, told them your character name, and then put up for auction a grey item that was priced at how much gold you had purchased. One of the vendors 'agents' would then go into the auction house and buy it. You ended up with your gold and as far as the game was concerned it was a straight up, if unusual, transaction between players.

[1] Item descriptions were colored, grey was "trash" and meant to be sold to vendor for cash, "green" were somewhat nicer/rare, then "blue" for very rare, and "purple" for epically rare, and "goldenrod" for legendary.