+0
I don't see a point with buying something that I could have done myself. Or

I don't see a point in buying something that I could have done myself.

Can somebody please tell me why the correct preposition is used in this case?

Thank you.
+0
PreciousJonesCan somebody please tell me why the correct preposition is used in this case?
Are you sure this is what you wanted to ask?

the point in -ing is the phrase you want.

I don't see the point in [going there / doing that].

CJ
Comments  
I think you will find that most native speakers (of AmE at least) will use "in" in this exmple. As to why--who knows, it's probably just an idiomatic use as is so often the case with prepositions.
 CalifJim's reply was promoted to an answer.