Vivian in the 2nd section of Onyx City allows you to shop at other players' Collectable Custom shops, as well as modify your own if you happen to own one.
To buy a Collectable Custom in this shop, do the following:
Unlike the Auction House, you are guaranteed to receive a refund for a Collectable Custom you bought here for coins or credit if the creator ever decides to have his or her work removed from the site.
Please note that, because of a small bug, all purchases before August 1st, 2014 are ineligible for this protection.
In order to get a Collectable Custom shop of your own, you need to first create a custom with your own unique art. After that, Ignis will let you know if he's interested in letting you be added to the User Group Artist so you can begin selling your own creations.
Assuming you have your own shop, just click your avatar there. The Shop NPC Images menu will be on the left.
From the Shop NPC Images menu, click to access the Shop NPC Text menu. Alernatively, click to return to the Shop NPC Images menu.
After clicking a creature in your shop, you have the option of giving it a coin price, credit price or both. Remember that the amount you're selling it for must be greater than 0. Click the button when you're done.
From the Custom Pricing Menu, click the button, enter the percentage off and then select how many weeks the sale will last. When you are finished, click the button.