"You said, "Now if the coins are in above average condition, like an MS68 or MS69 ranking, you will pay a bigger premium for the extra quality."
MS68, 69, and 70 are rankings for the coins. An independant group called "PCGS" I believe accepts coins from dealers and individual owners who want them graded...They inspect the coins thoroughly and assign rankings for quality (smoothness, lack of any miniature nicks or flaws anywhere on the coin, etc. Most of the basic coins you buy straight from the roll will be very nice coins and rank around "MS67" in quality. Very very few coins will rank a perfect MS70. These are the ones with a perfect stamp...the design is raised off the coin...Everything is very clear and smooth on the high grade coins. It is a very subtle difference overall, but the high ranked coins to end up being worth quite a bit more than unranked coins. Normal coins will be described as BU ("brilliant uncirculated") and tend to have some very very minor imperfections...nothing that stands out at a glance. I like regular BU coins. They are cheap, easy to store, and easy to exchange for reasonable prices. You may need to haggle over the price of a graded coin, since there really isn't an acceptable "standard" price for them, as there is for regular eagles.
All of the PCGS certified coins are sealed in a hard plastic container and given a serial number to verify their grade. If you do a search on Ebay for "MS70 gold/silver eagle", you should find several results...The coins are beautiful and virtually flawless...and sell for a pretty big premium over melt value. I don't recommend graded coins for investment purposes, but they certainly could make a nice (and rare) addition to your permanent coin collection, and an awesome gift for the coin collector in your family.
Oh, and "bullion" is just another word used to refer to precious metals. Bullion content = gold/silver content
As for the dates...older coins tend to be worth more than the newer coins, simply because they are harder to come by. This is not always the case however...It really depends on the number of coins minted and how easily they can be found on the market today. I believe that 1986 was the first year the gold coins were minted in the US and coins from that year are getting hard to come by today. I believe 1998-99 on the other hand had very large mintages of gold eagles, so you have a good chance of finding a 98 coin at or below what a 2003/04 coin would sell for today. It all comes down to their circulation. I believe you can find the mintage stats for every year somewhere on the US mint website to see which coins have the most/least mintage. I also recommend you check out "tulving.com" when browsing for deals. They tend to have good deals (low premiums) on older gold eagles when they get them in stock.
"And a question about buying/selling strategy - is it like stock where you buy low and sell high? If I bought gold five years ago, would I be thinking of selling now? In other words, should you skim profits when you can and wait for a new low to buy more, or just hang onto it?
If you do sell, do you have to report it and pay taxes on it?"
You can "trade" them if you choose I suppose. In today's environment however, I'm not selling anything. I think the dollar will keep sliding and the value of gold will increase as a result. Gold and silver do one thing well...HOLD THEIR VALUE. A one ounce gold coin will always be worth something...The little green rectangles printed by our government can be made worthless by running the printing presses all night, and by drowning the nation in debt. In fact, most of the gain in gold and silver prices has not been because demand increased...It has been because the value of the dollar has fallen off a cliff. The value of gold hasn't changed. The value of the dollars you use to BUY gold are what has fallen, causing the price rise. Gold and silver tend to hold their value regardless of what the morons in Washington or any other country do. The amount of gold/silver in the world is limited and finite...It rare by its very nature. This is what makes it such a unique investment. It will always be worth something. Every company on the New York stock exchange can issue a million more shares of stock tomorrow, dilluting the value per share of the company...Every government can print 10 billion more dollars of currency tomorrow...but the country will NOT be ten billion dollars richer...Everyone who holds the stock in those companies or paper money from those governments will end up a little bit POORER as a result. Governments can not "print" an unlimited number of gold and silver coins. They are rare by their very nature and will always be sought after items.
If you want to "trade" in the precious metals, you might want to consider some mining stocks that trade on the market...You can buy and sell them every day, and they typically have a move 2-3X what the price of the metal does. This has obviously been great lately...While gold has gone up 50%, gold stocks have ran up 150%. Of course, it works the same way on the downside so you have more risk in doing this. I don't like the risk so I have stayed away from this option. Personally, I recommend getting a small stash of gold and silver, finding a safe place for it, and locking it away for a rainy day. Don't spend all of your money on precious metals. First, I recommend you pay off any high interest debt you have, especially credit cards and car loans. Remember, killing your VISA balance is like getting an INSTANT and GUARANTEED 17% return with absolutely zero risk. Paying off debt should be the first step for anyone...Once you have major debts erased and save up a few extra bucks, it certainly couldn't hurt to add a little shiny stuff to your collection, accumulating a few silver eagles each month and saving them for a rainy day. I'm not a financial advisor, but this would is my personal advice.
Do your own homework.
Good luck.
And have fun collecting. :)