When you buy resources from neighbor do you take the card or just get access to the resources you need?
Just the resources.
The rules on page 4 define things pretty well.
Cards are called "structures", the brown ones can generate "resources"
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.
In red, there is a notice:
Important : the resources are not spent during construction. They can be used each turn, for the entire game. The production of a city is never reduced (the cards played are never discarded).
Obviously if you were to take the card, the production of the city would be reduced and this is prohibited.