Hello,often you read an airline orders xx new aircraft to renew the fleet or they completely switch from B0ing to Airbus or vice versa.Or many airlines tend to go bankrupt and the banks sells all nuts and bolts, inlcuding the whole fleet.Now I wonder where are all these "used" A318-A340 for sale ?Is there something like eBay for airliners somewhere ?

There's specialised companies that trade in used aircraft. Boeing also has some in stock I believe that they took off the hands of a customer in partial payment for some shiny new 777s (or maybe they've already sold them on).A good source would also be lease companies like ILFC.

Most are leased, not bought, then are treated as an ongoing operating cost and paid for (hopefully) with revenue. There are many aircraft brokers, and even the manufacturers sell used aircraft that they take back as "tradeins." The biggest factor in deciding cost is where the engines are in the maintenance cycle. There are mandatory inspections and maintenance (up to and including a complete rebuild) based on the number of hours the engine has flown, and as this is quite expensive an aircraft nearing the end of the cycle is considerably cheaper than one at the beginning.billg