I try to buy RL polos on eBay most of the time. Fakes are getting better at copying, but there are still a few things you can look for.
One fool-proof way you can spot a fake is the collar band on the inside of the shirt.
As you can see with this shirt, the band that is running along the collar is made from the same material as the mesh shirt. This indicates that it is a fake, as all real RL polos have a different material for the band. I would show you an example of a real one, but unfortunately, all close-up pictures of RL polos on eBay were fakes. Just check out the shirts you bought in Macy's or in another department store, you'll see that the band is made of a softer cotton and has horizontal ribbing running along it.
I have yet to see a fake that has this, probably due to slightly higher costs and most importantly, customers just don't realize it.
If you're considering a big purchase of RL polos, I recommend you ask for a close-up picture of the collar tag (so you can see the banding.) There are a few sellers that sell authentic polos, you just have to investigate, ask for better pictures and hopefully find a legitimate seller. I originally went for the dirt-cheap items and paid the price. Prepare to pay slighter higher prices than the other polos on eBay, but be happy that they're real!
Hope this helps!