Hi Nikhil,
It is not a thumb rule... and since the case is with buyer you cannot demand, you will need to alter your system to adjust and understand whatever he is sending and transform into the required format.
If the buyer agrees to send an IDoc [which is rare if he doesn't have SAP then you can create an Inbound Mapping for the IDoc and write the conversion your self or process inbound DELFOR with process code DELI FM IDOC_INPUT_DELINS_START
Any how a program is needed for the same to understand transform and process the file send by the buyer.
Regards