Reverse charge mechanism

Hi,

The seller is at Maharashtra state having GST No. while the purchaser and service availer is at Kerala without GST No.  would request you to clarify on the following points :

1. Is it mandatory for the seller to have GST No.

2. Can we charge it at Reverse charge mechanism by the purchaser

3. Few says that the billing can be done with with IGST charged and seller need not have GST No.

Can anybody explain in detail, please.