This help page explains our Send Stored Codes feature:
http://www.e-junkie.com/ej/help.selling-codes.htm
It would be up to you to obtain and paste in a list of PIN/Registration codes that would be suitable for whatever purpose you expect buyers would use their codes for. For example, if they would need to enter their code with some service, you would need to either obtain the codes from that service or provide the same list of codes to them, so they could match up whatever code the buyer enters at their end.
You can use a site such as www.random.org/strings/ to generate a list random codes, but those codes wouldn't do anything by themselves; you'd still need to provide those codes to whatever service the codes would be used with.