I feel lucky here since we got M6.Net (2 letters).
We have never sold any existing/registered domain. But we are reseller of Enom and allow our clients to register their domains. I won't say we are making much profit out of it, but something is better then nothing.
Client found it convenient too when they can register their domain name through their web hosting company, easy to manage DNS.
If you are a web hosting company and not selling/reselling domain registration, I will strongly suggest you to do so. It is also good for promotion.