It's okay(-ish) if he plants about 12 trees each Christmas after buying one natural tree. It takes between 8 to 12 years for that tree to grow, so if every natural tree buyer would plant this many trees each year, the species would be preserved. It's a way to balance it all out.
I know a guy who goes out and plants 30 more trees after he picks one from the market each Christmas. Of course, there is no way to know if he's speaking the truth.
There is also a way to replant the tree after using it, more complicated and too hard for most people, but it needs to be sold with the root still intact.
Maybe nole_no1 can tell us if he uses one of these methods, or not.