The eBay app (free) doesn't have seller capabilities, so if you're planning on making your first billion by selling stuff online, you'll still need to get another app like GarageSale or stick to using the horrific eBay website. The eBay app has very few reviews so far on the App Store, most of which are griping about the inability to sell items through the app.
The app is very useful for searching, saving searches, and watching auctions, and takes up very little real estate on your Mac desktop. I found it to be quick in terms of response time, much faster than what I usually see when using the eBay website with Safari.
The watch list under the My eBay tab in the app is quite interesting, as it updates about every five seconds and displays last minute bid changes without needing to manually refresh. If you're a Mac-owning fan of eBay and love to pick up bargains online, the eBay app is definitely worth the download.