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auction and bid just before the auction ends. The intent is to make the auction's last bid and avoid price escalation caused by bidding wars. While making the last bid is what characterizes snipers, a more important feature is that they enable people to participate in online auctions without having to dedicate their time to monitoring individual items or making bids at the most opportune moments.

While eBay is the most popular target, sniping programs can purchase products from any auction website, including Yahoo!, Overstock.com, uBid, or even official US government auction sites.

The sniping process is similar to that of the procurement bots described earlier. The main differences are that the clocks on the auction website and sniper must be synchronized, and the purchase trigger is determined by the auction's end time. Figure 19-2 shows a common sniper construction.

Figure 19-2. Anatomy of a sniper

Get Purchase Criteria

The purchase criteria for an auction are generally the auction identification number and the maximum price the user is willing to pay for the item. Advanced snipers, however, may periodically look for and target any auction that matches other predefined purchase criteria like the brand or age of an item.

Authenticate Buyer

Authentication of snipers is similar to other authentication practices discussed earlier. Occasionally, snipers can authenticate users without the need for a username and password, but these techniques vary depending on the auction site and the special programming interfaces it provides. The problem of disclosing login credentials to third-party sniping services is one of the reasons people often choose to write their own snipers.

Verify Item

Many auctions end prematurely due to early cancellation by the seller or to buy-it-now purchases, which allow a bidder to buy an item for a fixed price before the auction comes to its scheduled end. For both of these reasons, snipers must periodically verify that the auction it intends to snipe is still a valid auction. Not doing so may cause a sniper to mistakenly bid on nonexistent auctions. Typically, snipers validate the auction once after collecting the purchase criteria and again just before bidding.

Synchronize Clocks

Since a sniper uses the closing time of an auction as its event trigger, the sniper and auction website must synchronize their clocks. Synchronization involves requesting the timestamp from the online auction's server and subtracting that value from the auction's scheduled end. The result is the starting value for a countdown clock. When the countdown clock approaches zero, the sniper places its bid.

A countdown clock is a more accurate method of establishing a bid time than relying on your computer's internal clock to make a bid a few seconds before the scheduled end of an auction. This is particularly true if your sniper is running on a PC, where internal clocks are notoriously inaccurate.

To guarantee synchronization of the sniper and the online auction's clock, the sniper should synchronize periodically and with increased frequency as the end of the auction nears. Periodic synchronization reduces the sniper's reliance on the accuracy of your computer's clock. Chances are, neither the clock on the auction site's server nor the one on your PC is set to the correct time, but from a sniper's perspective, the server's clock is the only one that matters.

Obtaining a server's clock value is as easy as making a header request and parsing the server's timestamp from the header, as shown in Listing 19-1.

// Include libraries

include("LIB_http.php");

include("LIB_parse.php");

// Identify the server you want to get local time from

$target = "http://www.schrenk.com";

// Request the httpd head

$header_array = http_header($target, $ref="");

// Parse the local server time from the header

$local_server_time = return_between($header_array['FILE'], $start="Date:",

$stop="\n", EXCL);

// Convert the local server time to a timestamp

$local_server_time_ts = strtotime($local_server_time);

// Display results