Task Scheduling - Listening to data in the future.

I am working on a peer-to-peer e-commerce website where you can buy and sell items. Every time an item has been bought, it notifies the user with a notification. A user is able to create a "wish list" where they can post ghost items that are not currently being sold on the site. But when a user has uploaded a similar item it needs to be able to notify the user that the item is now being sold.

My problem is basically finding an effective method of doing this. My instant thought was to not check the database straight away for the items that are similar, and I would not be able to check into the future. I can imagine comparing all the items would a very performance heavy task.

So I think my only other solution I believe would be task-scheduling and check for the result at X period of time. I am not really familiar with scheduling and am pretty new to the idea. So was wondering if performing eloquent check in scheduled task was a good idea or not? I was just wondering if I was heading in the correct direction, or totally wrong?

Any advice or examples would be a great help, thanks. :D