I remember looking up info about that a few years back where a 1oz coin costs a hell of a lot more than the face value on the back. There’s a few countries that do that… it’s almost like better off melting them down.
That’s simply because they aren’t anymore intended to be used as currency. They’re just useful if you want to buy gold and don’t wan’t/ can’t buy an ingot.
Only the market value of the gold they contain is relevant. And as long as they’re in perfect state, their weight in gold, hence their value is perfectly known (*)(contrarily to the bit of gold you would get by melting them down. If you did so, you wouldn’t get any more value in gold, and the potential buyer wouldn’t know exactly what he’s buying, until the gold is graded and weighted).
Since the facial value is irrelevant, most (all?) countries minting gold coins still mint them with their original/traditionnal facial value (the french gold coin has the same facial value since 1860 or so, for instance). I suppose that they could still be a legal tender and that if you were foolish enough you could buy a round of drinks with your gold coin, and that the bartender would happily accept it.
(*) Though I believe the value of a (regular, not collectible) gold coin can be shlightly different from the value of the gold it contains.
In the case of older collectible gold coins such as the $20 St. Gaudens (issued in the early 20th century), the face value was indeed the coin value when issued. Now, of course, between the price of gold and the collectible nature of many of these coins, the value has increased dramatically.
The price of gold fluctuates too much to try and use it for real currency.
Because the gold coins are legal tender, they are covered by laws prohibiting counterfeiting. This gives the buyer some more protection. At least, the guy who sold me some golds coins claimed that. And if you can’t trust a salesman, who can you trust? 
Once it’s used as tender, does the coin become the property of the bar? Or can the bartender pull a $20 out of his own pocket (assume the coin in question is a $20 coin) and swap it out?
One of the main reasons to buy gold coins is their liquidity. A Canadian maple leaf can be verified with a few simple measurements and any coin dealer pretty much anywhere in the world will take them. Try walking in with an ingot. It will have to be assayed before you can sell it and you will pay the assay fees. If you want to have some gold, coins are by far the best way to go.
I think the bartender would have to be pretty much of a moron to even put the coin in the till. Personally, I’d buy it from the customer and then let him pay his tab, but I am interested in the legal position.
I seem to recall hearing about legal restrictions on a private citizen owning gold (other than jewelry or currency), is that still valid? Granted, this was in some of Heinlein’s “surviving opressive gov’t” stories written back in the 50s or 60s, and I’ve never really looked into it.
Actually, a quick google brought up this. If the dates are accurate (and it hasn’t been restricted afterwards), private gold ownership was illegal between 1933 and 1970, so that wouldn’t be an issue today.
On a question somewhat related to HeyHomie’s, you occasionally hear about people dropping $20 gold pieces into the Salvation Army buckets around Christmas. If they chose to deduct that contribution for tax reasons, do they claim the market value of the coin ($720 when bought new from the US Mint) or the $20 face value? Could the IRS make the case that they “only” gave $20? Or would it be counted as an item donation, not “money”?
In the USA anyway, I believe that the (law? executive order?) in the 1930s that banned the private ownership of gold effectively stripped gold coins of their “legal tender” status. So no piece of gold is now a “dollar”, or any number of them.