today I bought the current version of GarageSale and installed it via App Store.
After installation I tried to autorize with the ebay Server but the “autorize button” was deacivated. During installation and later in Preferences as well.
This is a known issue caused by Apple’s latest OS X update. The issue is fixed in the latest version of GarageSale.
However, Apple hasn’t released the new GarageSale update so far that’s why you’re probably still using an older version. Apple should release it soon, hopefully.