May you choose not to use your own resources and buy them from your neighbours instead?
This is relevant for cases where you would earn money when "buying" from your neighbours.
The rules say about "Resource Cost" (English PDF, page 4):
Some cards have a resource cost.
To construct them, the player must produce the corresponding resources AND/OR buy them from one of his or her two neighboring cities.
And about "Commerce" (same page):
Often, a player will want to build a structure requiring resources he or she does not produce.
If these resources are produced by a neighboring city – players sitting directly to the left or right of that player – the player will be able to purchase the missing resource(s) through commerce.
It doesn’t seem to address this case explicitly, and I can’t deduce it from this wording. I guess the "AND/OR" was meant for different cases (e.g., that you can combine both ways).