In Detroit, Michigan and Brandon, Manitoba there have been shops that will trade in older bikes and give a percentage off the sale of a new bike. Many of those bikes that were traded in were then given to the non-profit shop. The project in Brandon, Manitoba was a smaller scale deal so the same shop that would get us a few bikes also sold us tools at cost. For context, this is a ~45,000 population town with only two bike shops so I don't think they saw our youth earn-a-bike program as competition or a problem. I imagine they saw us as another way of increasing bike ridership in rural Manitoba which they would benefit from.
In Detroit the shops were more spread out. Also in Detroit a few of those shops didn't do trade ins, they just agreed to be donation drop off points for us because we often had people that wanted to donate bikes or parts but didn't have a way of getting all the way to our shop. The key to those relationships was communication and transportation to pick bikes up. For profit shops didn't want their storage areas filling up too much with bikes they weren't going to use. For better or worse in Detroit I think the customers (and maybe the staff) of the suburban shops got to feel good that they were helping folks in the city. The shops could also advertise that they had this partnership with us to get some more customers in the door. We would also do thank yous to them on social media which our volunteers/donors in the suburbs would see. None of those shops were near our shop because at the time Detroit didn't have many shops.
There are a few for profit shops in Detroit now that are super supportive and community minded. After I left they partnered in our annual winter bike-a-thon fundraiser as a warming station for riders and I believe some of their staff rode to raise money as well. Similar to what other people said, there would be many referrals between those for profit shops and non-profit.
-J