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First off, SQL Injection will be a major thing to worry about, especially when using obvious get parameters.
Second, you will want non-personally identifying info in the GET parameters. You will NOT want short ids such as 123456 as these are VERY EASILY guessed. A GUID (example 412dc535-03dd-4887-b702-02c8b85e8891, you would remove the – of course) is great for this.
Third, you will want some sort of basic verification that it is in fact the user that originated the request. Have each email have its own ID for the one click action, perhaps a GUID (long string of random characters), and make them expire after some time (a few days). Keep track of each email link separately and what they’re supposed to do, so that someone couldn’t just figure out the ID of someone’s ad and keep running it over and over.
~Christian