|The customer is not always right, and neither is your manager. There are real questions here regarding the shop's management and mission.|
When customers are interfering with the work of the staff and engaging in overly self-serving behaviors in a volunteer organization, one has to wonder who is managing whom. How would the founders of the shop feel if they knew that the same customers or friends are hanging around and constantly grabbing the best items for themselves? Perhaps they might feel that as long as the merchandise is being sold, this makes no difference. Or, they might be appalled. You need to discuss this with the manager.
If the objectives of the shop are to work as efficiently and professionally as possible, bring in many people from the community, let them have equal access to a broad range of values, and provide more than picked-over merchandise, then it is time to clarify the mission and develop and enforce some policies that support it.
For example, it may be helpful to have a policy that prevents newly-received items from being placed on the floors until they have been fully checked and appropriately priced, and this can take some time. If the friendly bargain-hunters sense that they can no longer quickly pick up the best items, they may just as quickly pick up and leave.