What do companies like Sephora, Ulta, Walgreens and Elite Labs have in common?

They all either manufacture their own brand of Vitamin C serum or they distribute it.

What to Look for in a Vitamin C Serum

Well for one, not all vitamin c serums are created equal.  Elite Labs' vitamin serum contains 20 percent more vitamin c then its competitor.  It also contains skin softners and other ingredianta that are simply good for you