If you want to be informed as soon as the prices are reduced, simply add the listings to your Watchlist.

Adding listings to my Watchlist

Tap on the heart icon while browsing through the results or open a listing and click on the heart icon at the top right corner. You'll be notified by us as soon as the price is reduced.  

You can find an overview of all the listings you're watching when you go to My Shpock > Watchlist.