Different catagories rise and fall. Bulk legos will be going for $6 a pound one month, and $3 a pound a month later. A welding helmet may go for $48 during the week, but only $31 for the one that closes on Sunday. One week there may be only two Miller Maxstars listed, with ten people fighting over them; but the next week there may be five, and you can pick up the beat-up one for $300 less than the ones the week before. So before I even start buying, I just pick out items that seem interesting, and watch them to see what the final price is for them. It doesn't hurt to also go out and price the same things new on froogle or pricegrabber, just to avoid being scammed.
So, now you're ready to bid on some stuff. If you're online a lot, and enjoy the thrill of the hunt, then the first style: sniping, may work for you. Sniping is a term applied to bidders that wait until the last 30 seconds before an auction ends, and then enter in their bid. What's the point? Well, almost nobody bids the highest price they would really pay for something, and if you outbid them earlier, then they have time to consider the item more and maybe raise their bid up further. So sniping is all about hiding one's intentions, and lulling others into undervaluing the item. It takes some work though, to hang around right as the auction is about to end and bid right at the last minute. I'll often intend to bid on something, and pull up the page about ten minutes before the auction ends, only to turn my attention to something else and not remember it tell its already over. You can also get out-sniped by other snipers which is annoying. Still, its a good way to save some money if an item is particularly hot and you have the time.
If your reflexes aren't that good, you can also try scavenging. Scavenging is searching around for an item that is mis-labeled, mis-spelled, mis-classified, poorly described or otherwise not where it should be; so that the competition is greatly reduced (or even non-existent). My first success, was a scavenging operation, where after watching and bidding on a couple dozen hitachi nailers, I saw one listed as a NV 83a (but the picture was clearly a NR 83a). The buyer also didn't post a picture in the index, so only if you went looking at the page would you realize his mistake. Instead of paying $120-150 for the item (which was where all the previous ones ended up), I paid $77. I also scored a $1,500 Miller welder for $425 because absolutely nobody else could find the item (it was listed in "Lawn and Garden") and so I walked away with the opening price. This win was a double success, as it had initially been listed at $500, and I had even considered it at that price, but had forgot to bid on it the first go around, so it was re-listed at a lower price.
Once in a while, I don't have the time to wait three months to find an item. Then, if the dollar amount is not too high, its time to use the third strategy: the slammer. I needed a better saw for cutting logs, as my old $79 dewalt wasn't up to the task. Since I've been using 12" and 15" blades, the straight reciprocating action tends to leave the sawdust in the middle of the log and bog down. They have nice high power orbital action saws at the hardware store, but I wasn't going to pay $200 for one, so I browsed around on ebay for a bit watching the action on the Milwakee's and a few of the less popular brands like hitachi and bosch.
Finally, after a few days, I found the perfect item: a bosch 11amp orbital saw, from a high rated seller, in less than perfect cosmetic condition (a lot of like-new items are bought by dealers or store owners to re-sell, so their prices are generally higher). About a day before closing, I scoped out the competition and placed my bid. The previous bid price was $22, so I bid $48. Turns out I cut it pretty close. The previous bidder had actually bid $32, so the new price settled out at $33 until right at the end of the auction, when a sniper showed up.
This wasn't actually a pro-sniper, because he bid once at $40 (which I trounced), and then he thought about it and bid again at $44 (which I still trounced). If you're going to snipe, the whole concept is to be the last person bidding, so if you want to win, you should bid as high as you're willing to go the first time to avoid the, oops, can I bid again in the remaining 11 seconds before the auction closes? So don't really know if chucklesva would have bid even higher if he had had the time, but in any case, he didn't and so I won.
The new saw is great, and I've cut up two cords of wood with it in about four hours (only breaking one blade). Now I have to split all of it. I wonder how much a log splitter would be ...