You can do that sometimes...as long as your key isn't bound to a user account (Battle.net), and as long as you have the patience to get it working (mail the cds, give them your steam account password, find them a crack, etc). Some of those restrictions are reasonable and some aren't.
My point is that good DRM can protect rights of both parties , but never does. Instead, if often restricts the consumer's rights in favor of the publisher. Steam comes closest to balancing interests, but falls short as well.
