Create an estimate for each rep listing the items they take with them, I assume they are taking inventory items (requires QBO Plus).
when they come back having sold/used some, create the invoice or sales receipt for the customer.
If the rep replenishes his stock before going back out, there is no reason to adjust the estimate
if you have QBO Plus you can set up a class or location per rep and tag the transaction with it.
class allows you to tag each item line in a transaction
location allows you to tag the entire transaction
(I thought QBO had rep but I can't find it)