The review site BuyGoldWithBitcoin provides reviews of online stores offering precious metals for bitcoin.

I made four identical purchase of an American Silver Eagle Coin from different online sources using bitcoin. One of them was such a bad experience that you'd probably benefit from reading about it before buying there.

http://buygoldwithbitcoin.com/goldsilverbitcoin-review-from-an-actual-purchase-2013-09-23/reviews/

The two with the best service and price were Agora Commodities and Amagi Metals. Those reviews are posted as well.