One of the most important concepts in English grammar is called parallelism. One case of this occurs when constructing a sentence with two clauses that share some words. You can often just say the common fragment once, but if you do, you need to ensure the clauses are constructed as parallels so the meaning is obvious. Seth Godin didn't do this properly in a post today and it annoyed me enough to describe it in detail.
More and more, we buy stuff where the buying is the point, not the stuff.
The meaning of the final clause in this sentence, "not the stuff", actually includes part of the previous clause, we just omit these words for brevity. If we insert these words into the second clause, you can see the problem:
More and more, we buy stuff where the buying is the point, not the buying is the stuff.
Using parallel clauses, the correct way to write this sentence is this:
More and more, we buy stuff where the point is the buying, not the stuff.
The same principle applies to using the same verb forms and prepositions in each parallel clause.