It really depends on the weapon platform.
Off the top of my head, a registered sear could refer to:
-an HK sear
-an AR sear
-an AK sear
-an FNC sear
-an Uzi sear
To illustrate, let's compare an HK sear and an AK sear. With the HK, the registered sear is installed in a removable trigger pack. Generally, you would be able to move the trigger pack from gun to gun. An AK sear, on the other hand, is installed directly into the receiver. It's going to be permanently married to the receiver it is installed in. A registered AR sear could refer to a Drop In Auto Sear, which could typically be moved from gun to gun. Or, it could be the traditional M16 sear, which is permanently married to the receiver via the infamous third hole. To sum it up, a conversion device that can be moved from gun to gun is going to be more valuable than a conversion device that is married to one gun. Reliability is also weapon dependent. An HK sear is probably going to last a lifetime, an Uzi sear is probably not. A registered sear that is damaged may be able to be repaired, if none of the original markings are affected.
Hope that helps somewhat.