Modification that closes the thread on sellers command?
 

Could someone develop a hack that allows a seller to close their thread when they do sell the product? I would like it so ONLY the seller has access to the button.

Other optional items;
-Put [SOLD] tag in front of thread title
-Option to re-open in case it falls through

It's already coded into vBulletin, you just need to enable / disable it depending on what you want.

In your case, simply create a "Sellers" usergroup, and then:

AdminCP -> Usergroups -> Usergroup Manager -> Edit [Sellers] -> Scroll Down -> Post/Thread Permissions -> Can Open / Close Own threads -> Yes

And viola

Yes, but I would like it in button form, and for it to put [SOLD] tag at beginning of thread name.

I would like to add, when the thread/item is declared 'sold' by the seller, the thread is moved to a forum of my chosing, as an archive.

You'll need a seperate php page for this.