How to tell if something is fake on ebay?

I want to purchase a new iphone screen and digitizer, however I know a lot of people like to run scams on ebay with things like this. I am wondering how I can tell if it is fake or not.
I have never personally shopped on ebay before and have no idea what to expect. I would like to believe ebay has some way of filtering out the riff-raff, but I know this isn’t a perfect world… (If it were, my iPhone would be indestructible!)