If you are looking at sites that are manually updated, such as Trulia, then they aren't as accurate anyway. It's best to move quickly to see homes that you see because days on market is very low right now. But it there was one in particular, and it happend to be a short sale, there's a possibility to put in a back up offer just in case that first buyer walks.
Reasons for "off market"
1. Sold to someone else. Closed escrow
2. Sold to someone else, Escrow opened.
3. Sold to someone else, awaiting short sale approval.
4. Short sale failed, now undergoing foreclosure - may be relisting in two to twelve months by foreclosing bank. Not available for offer until relisted.
5. Short sale withdrawn, seller got loan mod.
6. Listing expired or withdrawn, seller no longer wishes to sell.
7. Listing expired or withdrawn, seller waiting for higher prices.
8. Listing expired or withdrawn, seller rented it out instead.
9. Listing expired or withdrawn, seller plans to relist after giving a short rest.
Your best chance of buying something, is to look at houses that are actively listed at the time you are ready to buy. Looking at expired and withdrawn listings is a comfortable way to postpone making such a big decision. Whatever today's inventory is, it will be different in a month.
I don't know how listings are going in your area, but in Colorado, our listings are way down and our sales are way up. What that means for buyers is, if you find a home that you like, you'd better make an offer quickly. Many of our listings are getting multiple offers and/or are selling within one to two weeks.
Don't wait much longer to get serious. It could mean losing out on the lower prices that we still have and who knows what might happen to mortgage rates over the next year.
Good luck to you.