Why do bullion and non-US minted coins sell more cheaply than US mint coins of the same weight and metal?

US minted coins are tax-advantaged in the US in that you don't have to pay taxes on the gains.

US coins (and most foreign coins) have a coin (or numismatic) value for being pretty collectibles, a face value (that represents the lowest the value of that coin can ever be, since it is also legal tender), and a bullion value that represents the value of the metal in the coin. While the legal tender aspect probably never enters into the discussion of a coin's value (I don't think anyone buying silver coins at $34/oz cares at all that they can never, ever, drop below $1/oz), it does add a bit to the numismatic value--it is a real coin, a piece of history, and so on. Silver from a non-government silver mint is just a piece of silver--if it is worth any more than that, it must have a really cool engraving on it.

Some older coins have a lower content of the precious metal they are made of. I'm not in the gold coin league of investment, so I don't know the particulars of gold coins, but with silver the content changed over the years. With US coins made prior to 1965, most of them were 90% silver. When 'silver certificates' stopped being issued and the US switched to 'legal tender', silver dollars were minted only to hoard silver--not to be spent. Since there was no longer such a need for durability, 'silver' coins fell into two categories--silver colored coins that don't contain much more silver than a dollar bill, and silver bullion coins. These silver bullion coins are nearly 100% silver. If gold worked in a similar way (I don't know as much about the history of gold), the reason older coins are worth less is that they contain less gold than a modern coin of the same weight.