It's really simple, the oldest auction gets sold. And there's more supply then demand. If I was still botting I would'nt post this but since I'm not and you're all retarded... (Or just keeping it to yourself ;p)
So if you post an auction at 6 PM, it's gonna be the oldest auction after 36 hours, which means in order to get shit sold someone has to buy yours between 5.50 and 6.00 AM.
If you post your auction at 6 AM, it becomes the oldest auction around 6 PM, aka primetime. And it has a high chance of getting sold. So in simple terms, if you want to sell gold on RMAH post it between 4 and 6 AM. If you post at 7-10 AM there's lots of others posting theirs when they get up in the morning, before going to work and such.