May you choose *not* to use your own resources and buy them instead?
As stated by the rules (English Rules, p. 4), you're required to use your own resources first and buy the missing afterwards.


  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.


This is also confirmed by the "City" expansion rules (English City Rules, p. 10):


  A player must use their own resource before buying one from their neighbors.


This rule is stated under "Team Play", but it states "neighbors", instead of just "teammates", like other similar rules, so I assume it's a more general rule. In any case, the base-game rules are quite clear on that.