Cheapest is godaddy, but also the worst and crappy customer service I ever see. Hassle free is Namecheap. Not always the cheapest, but watch out for their coupon days. I will recommend them for domain reg and admin. NC has a own forum and you can list domains for sell.
Use some affiliates or adwords to monetize your unused domains. If you got 1$/day per domain its not bad
Next good date is EARTH DAY at 22. April you can transfer for just 5.99 bucks.
I post no link. But if you want some mail me