Our service isn't really suited to providing a different file download to each buyer. However, if you're just selling coupon codes, you could use our Send Stored/Generated Codes feature to issue each buyer a unique batch of codes from a list you'd provide.
Bear in mind that if you're reselling coupon codes from a third party, you must have written permission from them -- either in their publicly posted Terms/Conditions, or as a written license to you in particular -- granting you the right to resell their coupons. Most major retailers we're aware of specifically declare that their coupons are non-tranferrable, meaning they can only be used by the person they were first issued to and not given or sold to anyone else. Such retailers often contact us to complain about unauthorized resale of their coupons, which constitutes a violation of our Terms of Service as well, resulting in suspension of the offending seller's account.
That said, these help pages explain how to use our Send Stored/Generated Codes feature: