sales 3 lacs and purchase nil in GSTR 3b, no previous stock,...

not able to understand....what u want exactly

whether sale of goods or provision of services???
Supply of service without being purchase made is possible but same is not applicable for sale of goods.

waise bhi in gstr 3b you can just show exempt/ nil rated purchases not whole of your purchases......... probably if you do not have any stock n you also not purchased from any registered dealer....
you might be thinking ke purchase from unregistered dealer dikha dunga....... but in 3b there is a coloumn "inward supplies exempted" or "nil rated" whatever the case may be............. you need to show the amount in that........