Hi Evan- Yes, this is possible through the use of views.
In this case I assume you have a Transactions table & an Items table. When looking at the Transaction table, let’s say you add a link to a particular item on one of the Transaction records. In doing so, a linked record field is utilized on both sides of the relationship.
If you head over to the Items table, you can then create a view that filters out all of the Items record that have a value in the linked record field.
After doing this, you can head back to the Transactions Table & configure the linked record field there to only show records from the new view you created. You do this in the field configuration popup. Doing so will prevent new links from being created for Items that already exist in a Transaction.
Hope that helps!