They don't "pop up" on the website. Their hunt code becomes active and once they hit someone's cart they become inactive again. The better tags go almost instantly as there could possibly be 100,000 people trying for them. You have to be quick with your clicks and entering the information any faltering/mistakes will not work out in your favor.
Reissue list isn't as scrutinize or important as leftover day. Tags become available at 9am on leftover day which is 09:00:00. Where the reissue is "On Wednesdays, all hunt codes that were on the Preview List will be released for sale as close to 11 AM as the system allows." That said, this could mean 10:59:32 or 11:00:49 or any other time for that matter. It just says as close to 11am as possible