Yes... you will be taxed on your gains. If you cannot prove your cost basis, then you will be taxed on the full sales price.
Some of your selling choices: (1) Sell back to the dealer who sold the gold to you (2) Sell to a local gold coin dealer (some are a huge rip-off so shop a round) (3) Sell to local pawn broker (very bad price, you'll lose money) (4) Sell to cash-4-gold dealer (worst possible price, you'll lose lots of money) (5) Get the US Mint list of American Gold Eagle bullion dealers and sell to one of them. IF they refuse to buy American Eagles, report them to the US Mint and they will lose their bullion dealer license. In order to keep their license, they must make a market (both buy and sell). (6) Find a bullion bank in major city and sell to them.
Always send USPS registered mail with insurance for full value and require signed receipt be returned to you as proof of delivery. Wrap in foam or bubble pack for protection. Tamper resistant packaging is always a good idea to keep package integrity.
USA and Europe have no choice but to discontinue their currency. The dollar wont be the worlds reserve currency. I believe they both will go to gold standard. They would have to confiscate the gold because it would be needed for the reserve.
you could sell anywhere because most are going to give you the spot price best place to buy is the coin show in your area worst place to buy is online becuase of shipping charges and they charges like $70 over spot. hell you can sell them at the pawn shop anybody will buy them