Usually you'll want to go directly to the source most likely to be selling it second hand, such as the Ebay app (although you want to make sure to filter to sold listings to see what it actually sells for rather than what people are asking). Just googling something will usually get sites selling at the MSRP which can vary pretty wildly from what it's actually worth.

One of the things a lot of professional resellers talk about is that being successful in the business isn't about knowing what sells as much as knowing what sort of things are worth looking up and how to look it up. Overtime you'll probably be able to recognize certain guaranteed hits but being able to tell what's worth 15-30 seconds to type into the search bar will get you a lot of great resell targets.