If you notice, there are an influx of Windows Phone 7.5 Mango news. This can only mean that it is just round the corner. And for apps developers, they are busy updating the app to make full use of Mango goodness. Ebay is one of them.
According to WPCentral, the updated version is 1.13 (from 1.12). However, it is a totally different app. You will need to install it as a new app. I guess the logical reason is because most people (like me) are NOT on Mango yet and therefore, the app might not be compatible. Here is the video of the Ebay app on Windows Phone 7.5 Mango.