No, no, no...
I think you misunderstand...
There's really two types of sears - registered and unregistered. If its a registered sear, it is a machinegun all by itself. The sear is registered as a machinegun. Those types cost about $10-11,000. That type of sear is not a part you can buy from HK or get out of an HKparts kit. It is a specialy made and designed sear to work in a semi-auto HK type weapon - the trigger pack must be specially modified to work with this type of sear. Its a non-standard conversion.
The unregistered types are physically different and can only be used as replacement parts in original HK machineguns. That type is physically different from the registered type and designed to work only in an HK machinegun, not a semi-auto HK. These are cheap - like $20 or so, but are not machineguns and will not make your HK semi full auto. Additionally, because they are not registered, if you use one to convert an HK semi to full, you've committed a felony.
The big difference (in a nutshell) is that the registered sear is designed to work in a slightly modified trigger pack to fit in any semi-auto HK and legally convert it to full auto.