Search custom fields with multiple records
One way is to do an exclude search.

Create a smart group that has all the people who have borrowed a tool in the last 6 month.  Create another smart group that contains everyone.

Under Search => Custom Searches you should find the custom search "Include / Exclude Contacts in a Group / Tag".

Include your group that contains everyone.
Exclude your group that contains the people who have borrowed.
This should result in the people who haven't borrowed.