At the Bike Dump in Winnipeg MB there are no set prices for used parts. One of our goals is to increase accessibility to cycling and bike knowledge by addressing economic and other barriers. If we charged for used parts, many of our patrons wouldn't be able to afford them.
If people are using them to fix a bike in the shop we humbly remind them that we operate by donations, but don't really expect it. If they are leaving with a bag of parts we strongly encourage a donation, but there are always exceptions. When people do donate it is somewhat anonymous since we just ask that they put money in a drop box by the door rather than giving it to a volunteer.
We have a list of suggested donations but it only gets referred to if someone is unsure what would be appropriate.