I'm not that technical so please take what i say with a grain of salt
What if you first need to enter the domain(s) into a separate entry field when posting in the "domain name sales" sub forum?
So accompanying with the "Title" entry field you have an entry field called "Domains up for sale/auction".
This will then migrate the domains into a separate database of some sort and attaches the thread/post number.
Then when you do a search as Ian suggested the search function would go through this database and spit out the results when partial domain matches are found.
So instead of searching each thread the search will be done through a database of domains that where entered by the poster by using a separate entry field in the post editor when posting in the "domain name sales" sub forum.