As far as online shopping and ebay goes,
1. You can possibly use a seller's inability to spell to your advantage to get some good deals. Sites like ebuyersedge(dot)com (typojoe(dot)com, etc) will search ebay for common misspellings of keywords you enter.
Most interested buyers will never see these misspelled listings, reducing the competition and most likely ending price.
Ebuyersedge also gives you the option of saving your favorite searches, then sending you an email when an item is listed matching one your saved searches, giving you a jump on other potential buyers.
This works best with 'Buy It Now's, but is also effective with auctions.
2. Whenever you send a question to a seller, use the "Ask a question" link that is on a page of another item they're selling that you're not interested in.
Maybe send the seller an offer to end the auction early. All they can do is say no.
3. If you bid on an ebay auction, use a sniping service (hidbid(dot)com, auctionsniper(dot)com, etc.) to avoid bidding wars, and hopefully save some money by not drawing early attention, and not giving manual 'nibbling' snipers a chance to react.