I'm guilt of this. Did it a couple years ago. Same reason, bought an upper with no BCG. It was cheap. I sold the unused upper cheap. I should have sold it with the bolt, not everyone has the gauges to check head space.
I doubt headspacing would be an issue with quality US-made parts. Probably no need to worry about that.
I would worry about someone using cheap/imported parts from suspect sources just to make a quick $$. Too many AR-15 "armorers" out there these days.
Stella - my best girl ever. 11/04/1994 - 12/23/2010
Don't wanna get shot by the police? "Stop Resisting Arrest!"