I've been buying power & hand tools on ebay for almost 2 years and there are a few things I've learned that I hope most of you will find helpful. Many of these tips apply to all of ebay, not just tools.
First, if you come across an item that interests you, completely read the description and shipping instructions from the seller. Many sellers have become adept at conveniently "forgetting" and leaving huge gaps in descriptions or taking photos that may mask or hide flaws in their items. If you have questions about an item, ask the seller as early in the listing as possible to allow them time to respond, especially on big dollar items. Also, tools are heavy! Make sure you know what the shipping costs will be before you bid. This will save you some shock when you get the invoice.
Unless you are in a hurry, take your time and do some research on the items you are looking for. Prices can vary widely on the same item from week to week. Look for sales that close during off-peak hours, typically early mornings on weekdays (less snipers then). Weekend sales will have the most bidders looking and of course sellers know this so typically the widest variety of tools will be listed during that time frame. Be prepared to have snipers bidding the item you want way out of control. If you want something bad enough, be ready to pay through the nose. Have a maximum amount in mind and then stick to it.
"Lot" listings may be a great deal or, in many cases, a pile of crap. Sellers will usually include 1 or 2 big name items for key word searches to entice you to look. BEWARE!! I've been burned once by this tactic but never again. A typical listing would read like this: Huge Snap On lot!! However when you read the description it will say something like: Two Snap On screwdrivers (usually very old/worn out) with other used tools (in other words, no-name junk). Reputable sellers will provide a complete listing of the items in the lot. Or, they may openly admit to be selling a box of old junk. In any case, remember that you will almost always get what you pay for. In other words: cheap $ = crap.
Look for words in the description like: "as good as Snap On" or "made by Snap On for...". Snap On does make tools for other vendors and these might be good quality tools. But they aren't the original and you might become very upset if you thought you were getting an awesome deal on a set of Snap On wrenches only to discover when you open your package that they actually have the name "Bahco" on them. Read the fine print!
When buying tools listed as broken and you have visions of trading them for brand new ones, be aware that your local dealer does not have to warranty them automatically, since you didn't buy them from him/her. The tools are warranted by the manufacturer, not the dealer. Snap On, MAC, Matco, Cornwell, etc. will most certainly repair/replace (their choice) the tool at your expense for shipping to them.
As for the tools themselves, it won't take you long to see some will always run more expensive than others. Snap On will always fetch the highest prices. Their reputation for quality, dependability, and innovation simply doesn't decrease their value much from new to used. They also almost always have the largest number of listings. MAC, Matco and the others sold from trucks are certainly all good quality tools. A smart buyer will buy what they need from all brands found for sale here. Craftsman tools offer the most bang for the buck and if your budget is your biggest concern, steer yourself that way. There is always a massive variety of Craftsman tools sold on ebay and almost always at excellent prices. You can build your box in a hurry by buying these and of course there's a Sears almost everywhere and you know you can always return broken items to them with no questions asked.
There are several other questionable practices by sellers but if you take your time and learn these tactics, you'll save yourself a lot of grief and money. Most sellers are reputable and willing to make things right if you believe they've gone wrong. Try to find a few regular sellers and deal with them. As a buyer, your only recourse is negative feedback (for whatever that's worth) and/or trying to get the item returned, not a fun process. What it finally boils down to is: CAVEAT EMPTOR!!!