Random tips

Gixen.com will save you cash when you use eBay, but remember to put in the absolute most you are willing to pay, and use random numbers. For instance, I often put in numbers like $102.23 if the most I want to pay is $100 or so (including shipping.

Always look at shipping!
Any website comparison is going to be difficult to do apples to apples, because they want to hide the amount of shipping and handling. Many times they will make you enter your email address or shipping address before they will quote you on the shipping. Remember, you do not know how much the item costs until you finish looking at shipping.

Quality vs. Price vs. Condition
Many times a good price comes at the cost of quality, but not always. For instance, it is generally known that cheap shoes are cheap because they...well, are cheap. But you can still get great shoes for cheap on eBay, but you get a different condition. If you are buying an item that still has 90% of the usability left in it, why wouldn't it be a great idea to buy it for 1/2 (or less) the price?

Televisions.
Face it, since you're not going to stop watching TV you are going to always want one in your home. You can get a great price on an LCD or Plasma TV that has been used for quite some time, but WATCH OUT. Television sets have a usable life, and when that runs out you are going to have to replace the screen or some of the electronic boards and then you will have to make another purchase decision, so...avoid it. Don't buy a TV that you suspect is near the end of its usable life. Look instead for an off-brand TV that has good specs and compare them with new items and slightly used items (remember that people generally don't replace a perfectly good flat screen TV unless they think they can get a better picture with another one).

Caveat emptor (buyer beware).
Watch out for items that are too close to free or don't make sense at the price they are listed at. Nobody is going to sell you a working iPhone for $50--it's simply not going to happen. Especially watch out for items that pretty much everyone and their brother wants: brand name MP3 players, new console systems and other in-demand items are going to be price stabilized. This means that unless you are buying it stolen or through a connection from someone who doesn't care what they get out of it, you're pretty much going to be paying the same price as everyone else. Beware of links on Craigslist and beware of listings selling hundreds of bullion coins on eBay all at the same time.

Basically look into things that sound fishy. If you can, call the person on the phone and ferret it out. In some rare cases, you may find that the person who is selling the item is simply out of touch with reality and is offering an unbelievable deal.

There is a reason other people have not already purchased it!

When it comes to Craigslist and eBay items offered on a buy-it-now, there is a reason that it's still there. You need to understand that reason before you commit to it.

How about 100 Sterling Silver and CZ rings for $55.00 free shipping, BIN (Buy-It-Now) 3 days into the auction? (Average wholesale for items as described: $280-$450.)
Reason: It's fake silver.

New Nike inspired Airflex shoes size 10 for $19.99 with 5.99 shipping.
(Average retail for item as described: $180.)
Reason: It's not a Nike shoe.

32 inch HDTV (no brand listed, fuzzy picture, all it says is "works great" with a buy-it-now of $250.) (Average retail: $50-$3000.)
Reason1: It's not a flat screen, its a tube TV OR
Reason2: It's a moron selling it for his mother and everyone else thought it was the former.

You have to find out. It could be a great deal, it might not, but you have to figure it out.

Research, research, research!
No matter what you want to buy, the solution to the problem getting the best deal is either going to be either research or being a crack dealer. Either one will give you an edge over the average unmotivated person and can allow you to save some serious cash, though currently there is no law against research... so I would pick that.