Query related to retailer.

sabyasachi mahana (articled assistant) (28 Points)

04 December 2017  
If somebody who is not required to registered under GST(i.e within the threshold limit of 20lacs) whether he is required to take all the purchase invoice on his purchase from the purchaser. If yes whether he is required to maintain any books of account.
If he purchase from some local seller who is also not required to registered under GST, in that case how he will get GST bill for his purvhase from the seller. Is it mandatory for him to maintain all GST bill for his all purchase?