ACME offers a feature to allow staff the ability to sell member discounted tickets to a member with an expired membership. This Toggle can be set to Yes or No for your venue in the Administration menu in Backoffice. 

Navigate to Backoffice → Settings → Administration to change this setting. You have the ability to set this permission for users placing orders via Backoffice on the B2B tab, and for users placing orders via the Point of Sale on the POS tab. If you only wanted to allow it on one of those sales channels, but not the other, you could. Users must have Tenant Administration permissions to access this menu. 

In addition, you can set a Grace Period for Expired Members to allow members to purchase tickets online for a specified period after their membership has expired.