I usually prefer absolute (by the piece) auctions, garage sales, family-run sales, and just about anything to an estate sale - living or dead.
A regular estate sale run by a professional company after the death, illness or forced retirement to a senior center of an individual usually has very few, if any deals. You may see a few items priced right for your own collection, but generally they're asking only 10-25% less than retail on Day One for their items. Plus, having worked with a few of these companies, I can assure you that they usually know (more or less) what they are doing and will grab any major deals before you do. Anything you find is only there because every bargain hunter before you and the company didn't specialize in that item's area.
Some of them get pretty cheap the last few days, but they're still one of my least favorite places to go. By then, there isn't much to pick through and it still often isn't a hell of a deal.
Estates run by someone living are either a bit better or a hell of a lot worse. You have the benefit of the owner being there, but that's a double-edged sword. You can haggle with them directly and skip the middle man in most cases, but there's also a level of emotional attachment that keeps you from getting those items. I've talked people into giving me things, but I've also mentioned items the person didn't know were for sale and had pulled from the sale.
One way to potentially turn a profit with these groups is to drop by on the last day and leave your card all while pointing to that big truck or van of yours. Most estate companies are run by women or older people (frequently both), and they often don't have the time, resources or energy to dispose of what doesn't sell if they're expected to clean out the entire house. Three times I've gotten the contents of an entire house, post estate sale; twice for free (the third estate asked I pay $100 for everything).
Even then though, don't bank on what you saw when you got the stuff. The only time I didn't get screwed by the company was when I did pay for it. The other two times? Things began to disappear whenever the company was left alone, and it's sure funny how it's only valuables that vanish!
At least at absolute auctions, the auctioneer very rarely gets to pick before they sell - and it sells for whatever price is decided.
Bonus: A small estate sale story that still has me in a tissy -
When I bought the house I live in, it had been abandoned for a number of years with the entire contents of the house (including food) in it. As it so often goes with families, they're more than happy to let something rot away (and some things did, due to a leak in the roof) until someone else wants to enjoy it/profit from it. Before I was allowed to move in, they hired an estate company to sell everything.
Most of it wasn't especially valuable. A few nice pieces that were worth X here and Y there, but nothing unique or worth losing sleep over. With the exception of one Botero painting. For those who don't know Botero; good. His art style is that if a talented but troubled ten year old, but he's famous enough to have a wikipedia page and his art can sell highly.
Before the company went through, I asked to buy the painting and a few other things directly from the seller. They refused, claiming they didn't want to go against the contract they had with the company. No amount of begging or bartering would get me that painting, but I decided to show up at seven AM to be first in line for the estate sale.
Only to come in and find... the painting wasn't there.
But the owner was.
"Oh, hey buddy! I guess I should of sold you that painting," He says to me, "They (the estate company) told me it wasn't worth anything, and asked if they could give it to their daughter. Figured why not!"
The grin on their faces. It was out of a Botero painting.
(Unsurprisingly, they were one of the ones that picked through everything before giving me the remaining contents)