From a buyer's perspective classifieds are great - you can find what you're looking for, and know the price immediately. Auctions can be of more benefit to the seller than a classified ad because bidders will compete to deliver the highest price to the seller. In an english auction, the most common type, an item is listed by the seller with a starting price, and sometimes a reserve price. The potential buyers then place bids on the item. When the auction is finished, usually at a specific date and time, the seller is obligated to sell the item to the highest bidder, providing their bid was above the reserve price if one was set.
Dutch auctions are a little bit different.
Usually they are for dealers selling multiple identical items, often of overstocked or discontinued merchandise. In a dutch item, the number of items forsale is listed, and that many of the top bidders are obligated to buy either at their bid prices or at the lowest of their prices. While classified ads are pretty straightforward, auctions can be a bit tricky, and rules vary from site to site. Taking the time to thoroughly read the rules of the auction site you are going to use will solve a lot of confusion down the road. Another source of confusion and problems arise from a buyer spending their money to receive a package which they later discover is not what they expected.