It's the seller's responsibility to create a staff account for your before you're made the new store owner.
Normally, the seller is instructed to create your staff account after you pay Escrow.com. The seller won't receive the payment until you have confirmed that you have access to the store and have conducted your inspection period.
Here is how to create your staff account:
Look for the email invite sent from the seller. The staff account invitation should be sent to the email address you originally used to contact the seller from their Exchange listing.
Complete the Create Staff Account form. The account you are creating will have limited access to the shop for the time being. At the end of the transaction, you will be made the account owner by Exchange. For this reason, it's very important that you provide accurate information in the sign-up form.
Click "Create account". This will automatically take you to the admin panel of the Shopify store. It's from here that you can begin exploring the back end of the store.