[jimpudar]

Nothing stops them, it's just that it will be harder to convert into cash than if the gold was stamped.

For example, if I take a couple of Canadian Maple Leaf coins to any local "We Buy Gold" shop, I'll get very close to spot price in cash with no questions asked. The fact that they clearly came from the Canadian mint tells the dealer that they can trust they are indeed made of 99.99 pure gold.

If I come to the same dealer with a bar of gold I melted and poured myself, they may need to send it out to a lab for analysis to determine the purity, actual weight, if I'm ripping them off, etc.