The biases of paid "bots"
One factor that can make a big difference in a site is how much of the content is updated automatically and how much is handled manually, says Michael Wellman, director of the artificial intelligence laboratory at the University of Michigan.

Manual updating limits how recent and how broad a set of products the site has, Wellman says. On the other hand, quality control is higher, but so is the ability to skew lists in favor of advertisers.

And like a lot of other things on the Web, it's buyer beware, says Wellman. If you're searching for something on a shopping site, he says, "you can't be sure they're not missing things. That's especially true of sites that have relationships with vendors."

So how do you know if merchants are paying to be listed or to be positioned higher in the list? "You need to assume it's pay-for-play unless you find some evidence otherwise," Wellman says. "Those who don't accept payment are eager to point that out."

Shopbots work especially well if you know what you want. Once you've decided which TV you want, use a bot or two to help you compare prices. "They are useful for commodity items -- books, CDs, something I can get anywhere," says Amy Greenwald, assistant professor in computer science at Brown University.

"To be smarter, you have to be careful," she says. A bot may point to an e-tailer who "may have the cheapest price, but may have a long delivery time or high delivery price."

Tips for better botting
Regardless of which shopbot you use, you can get more from the process if you keep a few things in mind.

• If you want to find the best deals, don't forget to factor in the prices from local retailers. "Online is not necessarily the cheapest," says Clark Howard, consumer advocate, and co-author of "Clark's Big Book of Bargains."
  
   
• Shopping sites don't always automatically post the cheapest items first. "Many times your best price is on the second or third screen," says Howard. But most people are creatures of habit and don't go beyond the first screen. So either stick to sites that allow you to re-sort by price or be prepared to noodle through a couple of pages of results.
  
   
• Scope out the seller's track record. Some sites will give you feedback from others who have dealt with the same e-tailers, which is a useful thing, says K. Sudhir, associate professor of marketing at the Yale School of Management. Others also will explain how they assign their ratings.
  
  "The long-term rating of a site is more important than the price they offer," Howard says. Look to prevent problems upfront, he says, "because when you have a bad experience with an online retailer, it's so hard to get satisfaction."
  
   
• It's tougher to get accurate pricing information if you're buying multiple items. A bot can compare one price to another. What it can't do as well is factor in a discount or deal if you're buying more than one item, says Sudhir. You might get the best price on that new video recorder from the store selling it for $300. But another store selling the same camera for $350 might also give you the bag and tapes at a discount. So go to a couple of sites that promise the best prices and see if those deals get any sweeter if you buy several things.
  
   
• Shopbots can be great tools even if you don't use them to buy. Sudhir admits that he will buy through a shopping site if he's fairly familiar with the product. But many times, he just uses 'bots to learn more and compare deals on something he might buy later. "People use it as an initial guide and cross-check," he says.
  
   
• And don't get complacent. Just because you scored a good deal with a site last time doesn't mean it will give you the best deal this time. Going back to one site consistently is "a big mistake," says Howard. "Be your own shopbot. You want to try multiple ones each time."