Ok, I've noticed that it does not work for land loot or fish loot. A sand tiger shark costs anywhere between 200 and 290 gold. I know how much they cost because it says it on the right when you catch them, and I mastered fishing before the double gold was implemented, so I know what these guys are supposed to fetch you on the market.
And let me be honest...double gold just ain't working in fishing. It is like the first guy said, the gold splits. I catch a huge shark that would normally be worth 280 gold. The game splits that into two entries of 140 gold and calls that pathetic split double gold. If it was true double gold, you would fetch 560 gold for that 280 gold shark since that is double the value.
The same problem is happening to land loot, it is split, giving you exactly what you earned or one gold less, because oddly, when an odd number of gold splits, it can only split into even numbers, so one gold piece is lost. It kinda stinks.
But sea loot does seem to be working better. The only thing in this game that doubles right now is the gold from the chests you earn at sea.