The third party mod won't show up on the games list if you have deleted the whole mod folder, i just tried it and it worked fine.
I'm going to add information on how to uninstall mods from Steam to the third party mod FAQ now :)
Edit: Added info on uninstalling third party mods from Steam to...
That happened to me too, the best way is to just reinstall Steam, i haven't had any problems with Steam.exe and those files deleting themselves after that ;)
Right click on the Steam icon and press 'Browse Games', a window should pop up, then just press the button 'Register a product i already own' at the bottom of that window, it should prompt you for a cd-key then.
The person you're buying it from might have registered it to his or her own Steam...
If an admin has specified a contact adress (homepage/email adress) you can try checking the page for info (pages with forums usually have a specific forum part for hackers that people wants to have banned) (or if there was an email just email the admin).
It isnt Steam that should do that...