You can sign up very conveniently for a seller/affiliate account at anytime by going here:
There is no cost involved as it is completely free of charge!
Your seller/affiliate account is a separate account and different from a customer account where purchases are stored under. The dashboards of each, seller and affiliate, are both combined within this one account type. If you already have a customer account you can sign up with the same email address, or you can sign up with a different email address, if you wish.