Tip 1: Do a comparison, go online and find a picture of a same bag and compare.
Tip 2: Ask the person if they can send you additional pictures.
But for me, just looking at the bag, that is an authentic bag.
The reason it may be so cheap is because some people may just be trying to get rid of the bag to purchase a new one. I have purchased a dooney and bourk bag for under $50 that is worth $200 and the seller just had too many bags. But do more research in you are still not sure.