Feedback is an important part of the Reverb experience! To leave feedback for a buyer, head to your Orders page. From there, you'll want to select 'More Order Actions':
After you select 'More Order Actions', the option to leave feedback will become readily available:
The ability to leave feedback does go away after 45 days, so be sure to note your experience soon after the transaction has been completed!
You can follow the same process to leave feedback for a buyer of a Reverb LP order, as well!