Unfortunately, we have no way to notify a third party when their discount code is used to place an order.
An affiliate program is probably the way to go for that, and for buyer simplicity you could run each retailer's affiliate link URL through a link-shortener and put that shortened link on their poster as the only way to reach your site to place an order.
You could use your own sign-up link to register an E-junkie account on their behalf that's joined to your program, using an email address deliverable to you initially, then obtain their affiliate Hop Link URL and run that through the shortener, then change their login email to turn the account over to them. Repeat as needed for each other retailer/affiliate.