User properties are user-definable text blocks specific to an auction template. You can create them in the Properties Inspector:
These properties can be included in the item description with GarageSale's design language. If you are selling almost similar items which only differ in a few aspects, e.g. T-Shirts with different colors, you can now use the very same description in all your auction templates. Instead of inserting different color names in possibly several locations of your item description, you include your own color user property in your description. This color user property can be conveniently changed from GarageSale's inspector window for each template of your items.
You can paste the language design language command to include a user property into your description from the auction preview's context menu.
The embedded user properties will not get resolved in preview mode. Instead you have to press the Control and Option(Alt) keys on your keyboard to temporarily resolve them and see how your auction would look like when posted to eBay: