What is the best way to import cashbook postings from a third-party sales system into QuickBooks to reconcile with the bank?

Hi originaltravel Thanks for reaching out to us here on the Community. What format are the sales recorded in at the moment? If you have the bank connected to your Quickbooks account then you could add the transactions in as sales receipts rather than importing the transactions and then matching them to your bank transactions. 

I have included two links, the first informs you how to categorise/Record money-in transactions as bank deposits or sales receipts and the second informs you how to import sales receipts: https://quickbooks.intuit.com/learn-support/en-uk/help-article/import-transactions/import-sales-receipts/L5BrcgXyh_GB_en_GB?uid=m1f3tc20