If you don't have some basic information ready to hand when looking to buy Gold or Silver on eBay you WILL pay to much.
Can you spot a fake?  Do you know the current market prices in the "real world"?
As I'm writing this on the 12th of June 2009 the precious metal markets are closed but I know that the last price on Gold was between $ US 938.30 and $US 939.30 for an average of $US 938.80 and Silver closed at $US 14.83 there are a lot of people paying $US 18.00 and UP for commonly available 1 0z bars and rounds right now on eBay because THEY BID TOO EARLY AND TOO HIGH!
First, don't be too eager to just jump in and bid on the first thing that looks good to you.  There are a lot of professional sellers who know how to craft a good looking listing that doesn't tell you everything you need to know.  At a minimum you need the weight, either actual weight for 0.999 pure metal or the weight of precious metal for alloys such as Sterling Silver or any coin silver.  If the ad doesn't specify 0.999 pure then assume it is an alloy.
You also need to know what the current prices are on the exchange and/or spot markets for the metal you are buying.
This is the MINIMUM you need to know in order not to overpay.
Where to find this information?  Current or last bid (if the markets are closed) prices are available from Kitco ( use a search engine to find it, sorry, eBay rules) in real time.  You will also find their buy and sell prices for precious metal in various forms from fabricated bars and ingots to bulk coin.  You can also check with American Precious Metals Exchange.  They have extensive listings for coins from all over the world.  Make notes as their inventory changes and that key piece of info on that 10 Kroner (Silver) from Denmark might not be there when you actually need the information.
Confused about the alloys?  Many people think that Sterling Silver is pure silver.  It isn't.  It is an alloy of 92 1/2 % Silver and 7 1/2 % Copper.  Makes a big difference in the price you should pay, right? Most U.S. Coins were minted from an alloy of 90 % silver and 10% copper until 1964 (last year) after that, half dollars had 40% silver until 1971.  Everything else is "copper clad" except for the .999 fine collectors coins such as the Walking Liberty Silver Dollar.
Mexican coins are usually .720 Silver up till 1956 then the silver content varies depending on the specific denomination and year.  One very popular coin on eBay is the 1968 'Olympic" 25 peso.  This coin contains 0.5209 oz (troy) of Silver.
Canadian coins have the same story except they are 80% (0.800) silver the last year for real silver dollars minted for circulation was 1967.  There are a lot of commemoratives with varying weights of metal, if you are interested in Canadians do some homework.  The 'Maple Leaf" is the Canadian 1 oz (troy) .999 silver coin currently being minted.
What is this troy business?  Precious metals customarily are measured using the troy ounce instead of the US customary ounce which is from the avoirdupois system of weights.  Go to Wikipedia for a complete discussion of weights and measures.  The basics you need to know are that a troy ounce is 31.1034768 grams or 480 grains and a US customary weight ounce is 28.349523125 grams or 437.5 grains.  The other key piece of info is that a troy pound is 12 troy ounces or 5,760 grains and a US customary pound is 16 US ounces or 7000 grains.
Get to know your local market where you live.  I have one coin dealer that will sell me as much silver (in either rounds or bars) as I can afford from an average of $1.25 over spot rounded up to the next quarter amount.  Example: close on Silver was $14.87 + $1.25 = $16.12 rounds up to $16.25. I don't have to pay any shipping or wait for delivery.  This is metal in hand in exchange for the fiat money Federal Reserve Notes that everybody carries around (backed by the "full faith and credit of the United States", no hard metal backing, exchangeable for another similar piece of paper only).
Begining to get the idea that a lot of people are paying a lot of money for the artwork?  This is OK if you are trying to build an art collection or are collecting coins, rounds, and bars as a hobby.  If you are buying as a hedge against inflation or as a long term investment you need to shop eBay with a calculator and a notepad or you WILL pay a lot more than fair market value.
Don't forget to factor in the cost for shipping.  It costs less than $2.00 to ship out a couple of silver 1 oz rounds or bars using First Class Mail.  Tracking doesn't cost that much extra on first class and insurance is whatever you can negotiate with your seller.  Some sellers are asking as much as $6.00 for a single coin!  Even worse some sellers insist on using UPS which is horrendously expensive (and slow) for items this size.  You can ship up to 13 oz (US ounce) as a First Class Package, I know as I have been using this for my eBay sales for several years (lots of manila envelopes).
I hope this information is helpful.  If so drop me a note thru eBay messaging.  If you find any errors, PLEASE let me know so I can try to correct them.  Thanks v_and_t_nut.