Provided that salvaged parts function as intended, they can be used to repair an oven. Any critical components ought to be tested individually prior to being inserted into the system. Doing so minimizes the potential for damage to other components in the system.
Though doing so may violate the warranty, most oven problems can be fixed using salvaged parts. Ovens often utilize rather simple control circuits which can be easily analyzed and repaired. However, ovens operate at high temperatures and directly interact with high voltage electrical signals and natural gas lines, so any repair work should be handled with extreme caution. In some cases, it's better to call a professional.