Its not hard really.

First if you have active moderators on a site you have two sections. One current and one sold. Move the post when it sells or is old.

The second one requires addons to the site but with vbulletin thats easy enough. You run an addon that auto moves the thread to an archive section x days after thread creation. This can be labeled sold/inactive. With this setup only new/active for sales are in the for sale section. One addon a site was using would send out an email to the thread owner periodically asking them if the item was still for sale. If they failed to respond it was moved. I dont remember what that addon was but it was cool.

I host a bunch of sites like this so I know it works.

I had seen somewhere that someone (I think an admin maybe) mentioned they wanted to close old threads. Thats real easy. Most of the sites I host run an addon that auto closes/locks old threads to keep people from dredging them up.