Go to the set's checklist page, and find where it says "See any inaccuracies?" Click the link, and request the missing card be added there. (You can add a link to the forum thread in the request to help whoever does it so they can see it.) That's the best way to get these buybacks added. Those requests go into a queue, forum posts get lost.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).