I used another service (SnapFish) because I think their pricing is better on a lot of things, but I believe you can probably create your user name and password, and then create your calendar and keep the artwork available on the site. Then you would just share the login info here and let everyone log in and buy their own items.
There may also be some sort of a “public link” option that you can share where you wouldn’t have to give out your personal user information.