Right now eBay employs two different system for specifying the attributes of your item:
Community-driven item specifics
eBay is right now rolling a new item attribute system, called 'community-driven item specifics' or 'custom specifics'. Instead of filling in the the attributes (also called details) eBay selected, the new system lets you whatever attribute you think matches your item best. eBay will suggest possible attributes and proved pre-filled values you may or may not use depending on the category you are selling your item in.
This system will eventually replace the old attribute system in all eBay categories. For a list of categories that support community-driven item specifics, please refer to this page: List of eBay categories with custom specifics support
To use the new system you need at least GarageSale 3.4 or a later version.
Select a category that supports the new system for your template in GarageSale's category chooser. GarageSale will display whether a category supports the new system at the bottom of the window (leaf categories only).
You now can start editing your item's attributes in GarageSale's attribute panel (click on the magnifying glass next to Attribute Sets). You can either fill in the details suggested by eBay or add details on your own.
Please note that in most categories a mix of new custom specifics and traditional attribute sets is used.
Traditional Attribute System
With the traditional attribute system, eBay will choose a certain set of attributes for a given category. You can provide values for these attributes or choose from a value list provided by eBay. However, you can not add any new attributes to your item. Some attributes are also marked as required.