Atif,
In simple term It does not matter either manufacture or Selling In SAP plant is just plant. And it needs to record the transaction so you need to have account assigned to respective transaction.
so the selling plant it still need to record the transactions,Plant A did not receive any error because it might have assigned in OMWD. Can you confirm if Plant B is assigned in OMWD? As Each plant in SAP should have account determination for valuation area.
Thanks
Hrusikesh