It depends on the purity of the gold / silver. The gold and silver coins mentioned in that thread are bullion which means they have a high content of gold. For example a 1 ounce gold coin would be 99.99% pure gold, and maybe 0.01% other metals.
As opposed to the metal chains which would no doubt be far less pure and have a higher percentage of other metals mixed in.
Some people buy gold and silver from ebay...but there are other ways. It would depend on where you live, but you may have a local coin supplier, or else you will probably be able to find a quality supplier online. If you buy from ebay - unless you know what you are looking for - you could be taking a risk as to whether the coins were legit. For example, many of the Chinese Panda coins are fakes.
However if you stick to the recognized coins; American (Eagle’s), British (Britannia’s), Canadian (Maple’s) and buy from a reputable ebayer or supplier you will almost always get a quality item. Also always avoid buy from China, Hong Kong and other eastern countries, they will 90% of the time be selling fakes.
You ask how can gold be worth $715/ounce. But what is the value of a $? Last year I could buy $1.60 for £1...todays I could buy nearly $2 for £1. Value of currency is relative - it is also only as valuable as people think it is. Basically a dollar is only valuable to an American, or to someone who can trade it. A lot of countries are dumping the dollar...so it has less worth, because people no longer believe the dollar to be valuable. On the other hand, every country in the world believes gold is valuable. So therefore it is valuable. And gold will increase in value way beyond $1000 per ounce.
About buying precious metals...there are a number of ways you can do this. As you noticed most people in the "Buy Silver" thread physically own their metal. Owning shares isn't really worth it, because they only have value based on the stock market - which can instantly vanish overnight. Physical metal is a different matter as it retains its value independent of the stock market.
So - what to buy? You can buy what is called "junk" silver. These are older coins, that contain 50% to 75% pure silver per ounce. This is a relatively cheap method of buying silver. The important thing to bear in mind would be to buy coins that are recognizable within your own country (otherwise you may have trouble selling them later on). eBay is perhaps one of the best places to buy this form of metal. You should be able to buy these at roughly the current price of silver
Another form of coin to buy is modern Silver or Gold Bullion. These generally range from 95% to 99.99% purity in silver / gold per ounce. Most of the modern coins clearly have their weight and purity marked on them. Canadian Maples are sometimes considered as perhaps one of the most pure of the bullion coins, though they are also more expensive. Buying these from a reputable dealer is probably your best bet. You could buy individual coins, or bulk buy. Some dealers sell discount packs of 10 or 20 coins. Bullion coins cost just above to going rate of silver / gold. Generally $1 or $2 over the spot price for silver, and around $10 to $30 over the spot price for gold - dependant on dealer. However you would also be able to charge this same premium if and when you sell the coins on.
People also buy ingots or bars of gold and silver. These are usually 99.99% pure so are quite liked for investment potential. Though they are not really as collectable as coins. You can buy these from eBay and from dealers. However they are becoming increasingly harder to get hold of.
The bullion form of silver and gold comes in various sizes. You can get 1/20 ounce of gold, or 1kg bars etc. All is good. Keeping the metals in your possesion is also a very good idea...as it will always be on hand. Of course if you are after buying a tonne of the stuff it is a different story.