I've done quite a bit of shopping on Ebay since I became a member in 1999.  People usually come to Ebay to get bargains, but one has to be careful.  Prices for the same item vary a lot, so it's important to know the value of the item you want to buy before you bid.

For example, if you want to buy coins, check an official source such as the Red Book or Blue Book to see the current value of that coin.  Once I spent a fair bit of money on a roll of "poor man's doubled die" 1955 Lincoln cents, only to find that that is not a die error and isn't worth anything more than a regular 1955 penny.  If you are bidding on a key or semi-key date coin, make sure you know how to tell if the coin is real or fake.  Don't buy coins if the auction listing doesn't include a picture of the specific coin you are buying.  Grading is very subjective, so you want to be able to see a picture of what you're buying before you bid.  You don't want to spend a bunch of money on a coin that's not the real deal or is of lesser quality than you are looking for.

If you are buying electronics, such as an iPod or computer, look up that item on the manufacturer's website to see how much they charge for it.  Recently I was looking at iPods on Ebay, hoping to get a good deal.  Just out of curiosity, I decided to see what was the highest listed price for an iPod Video on Ebay.  I found three being sold in Buy It Now auctions for about $600!  From the Apple website, anyone can get the exact same item, an 80 GB iPod Video, for $349 with free shipping, and can even get their name or a message engraved on it for free.  A 30 GB iPod Video costs $249 from Apple, also with free shipping and free engraving.  I was able to get a 30 GB iPod Video on Ebay for only $230, which included shipping and shipping insurance.

The point is, be careful when you shop.  You can get really good deals on Ebay, but make sure you know the value of what you're shopping for, or you could end up paying more than the item is worth.  A lot of sellers on Ebay either don't know the value of what they're selling, or they are simply trying to make some extra money by raising the prices.