It is possible to select any attribute in Description settings or for Item Specifics. If attribute is not presented in product, then it will be ignored.
However, for the fields which are required for eBay (price, shipping cost, category,etc), the only common attributes, those which are presented in all attribute sets are shown in dropdown.
Please, note, that for Price and Cost only attributes with Text Field and Price Input Type are available for selection. For Weight only attributes with Text Field and Weight Input Type are available for selection.