I don't care for Orbitz for the very reason you listed. On 3 separate occasions, I tried to book a fare they listed only to be told that it wasn't available. I did email them about this and received a reply that basically amounted to "sorry that's the way it is").
After I did a little research I found out that the first display you see is basically a list of published fares. The "second" dip (after you try to book) actually looks for seats available at the listed price. Since they are now charging a $10 fee, I usually just use them for quick checks/comparisons and then hop to the specific airline's web site when I am ready to purchase.
I did book a hotel through them and found it to be a farily easy, painless process. That was last summer for a November stay. Since they are owned by a group of airlines, I have no fears/safety concerns about purchasing through them. I'm absolutely sure you end up with a legitimate ticket/hotel/car reserrvation. I just don't like the fee and what I consider to be the "bait and switch" way they list the fares. JMHO. YMMV.