Here's our perspective: everything that comes into our shop is available for no money. If someone wants a bike: volunteer 2 hours and take our BikeWell class; if someone wants a part: take it and use it. What we have found: more money comes into the donation jar, on average, than when we had very specific 'suggested donations.' The jar brings in $20-$100 a day.
Addressing parts miners: we talk to people in the shop and create a community atmosphere, so people understand 'take what you need, leave the rest for others.' There is occasional abuse, but we decided to live with and address that on individual basis.
Once every two weeks or so, we will take a complete high-end bike or part (mainly a nice shock) to a bike friendly used gear store. He takes 20%, gives free cycles the rest.