I use a combination of ways. Since people spell things differently or abbreviate items, you have to use different techniques to discover what you are looking for.
Browsing the titles is helpful if people classify items appropriately. Secondly, the specific forum search scopes the search to the content for the dimension you want. Third, you have to troll each of the posts to see if what you want is in there.
I would like to see the following:
1. Support in the search engine for exact phrases.
2. A mechanism for recording selling prices that requires both parties to confirm the item/amount to show a current market price for the item in game. (?Stock ticker in a Shopping channel?)
3. Ability to import directly from the "Helpbot" store. (yeah, cut and paste is easy but lots of folks don't know about the store)
4. Ability to have sold items "tagged" in the posting text so default search excludes hits on those items unless overriden.
5. Have a "watched for" item feature that will send a notification if a desired item is posted.
6. Have the forums available via RSS publishing.