We don't have any system to generate or check the codes we issue for you. We only provide the buyer with whatever codes you choose to provide, so it's up to you to generate and check those codes in whatever manner you choose.
You can either Send Stored Codes, where you provide us with a list of pre-generated codes, and we issue one code to each buyer, or you can Send Generated Codes, where we would hit your keygen URL to obtain whatever code it generates for us to issue to the buyer.
Once the buyer downloads your file and obtains whatever code we issue to them, the rest is out of our hands. If your file is a software app, then that type of file would typically have its own, built-in routine to request a code, check it for validity against your own online database, register it as "taken" so another copy of the file can't validate the same code, etc.