I can buy PMs with cash, however more and more cities are requiring these stores to copy IDs. Selling PMs requires an even greater likely of having to identify yourself. I can only see the privacy of such sales failing even more in the future.
I have read about mixing services. Am I accurate that if I make a one time wallet to receive money, and then send it through a mixing service that I can the use the bitcoins without fear of them being linked to the original transaction? It seems to me that the mixing service could decide to just keep your money, has that ever been an issue?
I don't think there is a need to use a mixing service.
If you trust the buyer of your gold to not make your transaction public, noone will know you have received the BTC in exchange for some gold. There's no public record saying: "address 1Whatever3498098234934982 received 1000 BTC for 7 Oz of gold", people just see "address 1Whatever3498098234934982 received 1000 BTC from 1AnotherAdress2039840928349"
If you're buying gold, you'll have the problem that the seller of the gold (say an official site like coinabul.com) will probably have to keep records. You're probably better off buying locally using cash if you want no paper trail and the bitcoin transaction is the least of your problem in this scenario.
Selling metal for bitcoin seems pretty hard to do "over the counter" (like in #bitcoin-otc). You'd have to spend a whole lot of time building trust first.
I recently used bitmit.net to sell one silver eagle (for fun basically). It worked out well, but I don't think I'd sell an ounce of gold online. Too risky.
I would however probably deal metal vs. bitcoin in person (meeting physically) if I had the means to check the coin/bar for authenticity.
@2weix: I'm sure you keep records of your buyers, right?