Why do search engines like Kayak, Yahoo travel, orbitz etc. keep showing unavailable or sold flights?

It's getting so frustrating because every time I click on a price it'll take me to the website and say that the flight is no longer available or due to the airline availability it is now available at an insanely higher price.

Is there a search engine that is more reliable in its listings?

I have to correct the previous answer! Search engines a Kayak do not want to rush you to buy. They are NOT a travel agency, so they don't take commissions when you buy. They forward you to a travel agency or flight company.

Never the less - I would try the search engine Momondo.com. They have a button called "live search" when you make an ordinary search. In this way you can make sure, that it is the current price :)

This has a lot to do with airline fare controls. Only a limited number of seats on any flight are available at the lowest price. Some busy flights have NO cheap seats, while others might have only a couple. Flights at off peak times (like very early, very late, or mid-afternoon) may have more cheap seats. Now picture the way it works. When the actual flight is requested, the website you are working on has to communicate with the airline website to see if seats are available. The extra time needed for the "outside" website to "talk" to the airline's system could easily be the time that a customer logs directly into the airline's website and grabs the last available seat at the low price.

It's about advertising. They show the unavailable or sold out flights because they want you to think that you have to buy the tickets now; to give you a sense of urgency. Usually there are plenty of flights available, these sites want you to buy through them NOW so they would have you believe otherwise. However, since it is the holidays, it would still be wise to book your flight as soon as possible.