Not receiving the money in 12 hours is fine - but the buyer can at least show that he has initiated the transfer within 12 hours.
Actually it depends on bank. HDFC bank will not allow you to do any transaction with new payee for 12 hours. ICICI will allow you to transfer instantly once you have verified payee via 2FA. SBI has I guess 4 hour cooling period. Now you must club it with NEFT payment window.
The simplest thing will be give rate at the time of transaction when user send you confirm email after NEFT transaction. In case you need buysellbitco.in historical rates page or api let me know, we will be able to give that quickly
Cheers