Do a bank account need to be added to User Account (Seller/Buyer)?
Only if you want to credit or debit their bank account.
Is it necessary to Add a bank a/c to any Merchant (Seller) Account for performing any transactions(i.e credit).
No, it is not necessary. You can charge a credit card without setting up bank accounts. If you want to move money out of escrow, you need somewhere to move it, which is most likely a bank account.
Is it necessary to Adding a card to an account to any Buyer A/c for debit amount from buyer?
If you want to debit an account via ACH, you do not need a credit card on an account.
Is it necessary to Underwrite an individual to a Merchant account for performing any transactions(i.e credit).
If you want to payout to a merchant, you have to underwrite them as a merchant.
Do I need to get both Buyer & Seller registration information and their corresponding Bank Account & Credit Card information for performing transactions?
No.
If you need more assistance, hop on the IRC channel https://docs.balancedpayments.com/current/?language=bash#support