National garage sale directory
  Users can list individual items for sale at their garage sales
Classifieds
Automobiles
Jobs
Real Estate

Other items?
A system to allow trading?
A system to figure out complex circles of item exchange based on the needs and offers of many simultaneous users? (would this be any more effective than money!?)

"unidentified items"
put an item in a category + subcategory, with picture, just for someone to come along and tell you the model number / species variety / etc.

register an item you own with the website where it can stay indefinitely or until you revoke it
anyone who searches for this item can see that you have it and give you an offer for it
you may also list your object with the amount of money you'd require to sell it

categories and/or tags up the yin-yang for real estate and possibly other types of objects for users to do fine-tuned searches for what they want

have a registry of things you're looking for on the site and automatically get emailed whenever a thing becomes available

have bidding like on ebay?

ability to bid on many of the same item at once and retract all but one when you win one? (and ability to retract bids in general?) an item with a retracted top bidder would default to going to the next highest bidder, at the price just above the next top bidder before him/her. unless they retracted too, then it goes down the line until a stopping point.

for searching, allow grouping with () and AND and OR like in google.