The following is for complete sets:
First of all, I get a feeling for what currently is a "bargin". Prices for sets vary a lot throughout the year. Therefore, I do the following:
Search term: lego "set number"
Search options: auction, soonest end time
After a few auctions have ended, I already know what would be a good price for a given set. Then I save the search and look for promising auctions, with patience. Usually I don't follow "MISB, NEW etc." because I want to build, not put away sealed. I also watch out for "no minifigs" because that reduces the price significantly. I also don't care about the original box. Building instructions, on the other hand, are important to me.
Sometimes, missing parts come in handy when you know how to replace them (or already have them). Even simple missing parts tend to reduce the final bid.
Note: After a couple of months your bargin price may not be a bargin any more: Once, I didn't buy a used 42009 for around 110 Euros, now it costs at least 160. Too much patience is not good if you really want to own a particular set. :)
Note 2: I have never understood the "Best results" default. "Best" means that many other bidders are already aware of that particular item. Not good for bargins.