Transitional credit of cenvat duty on goods purchased

Yes the dealer is eligible to take credit. If he has excise invoice then he is eligible to claim entire tax paid as Transitional CGST credit as per section 140(3). Further, the dealer has to file TRAN 1 giving the details of goods held on 01.7.

Yes, a dealer can take a credit of Central Excise Duty in respect of stock lying as on 30.06.2017 even though he is not a registered dealer under excise in the earlier regime. Further, even if document does not contain excise duty still the standard credit @ 40%/ 60% shall be available. Refer section 140 of GST Law.