Final answer:
Marvin should consider overall value, including the condition, features, and potential additional benefits of each car, beyond just the price difference when deciding which used car to buy.
Step-by-step explanation:
When deciding which used car to buy, Marvin should consider not only the price but also other factors such as the condition of the vehicles, features, warranty, maintenance history, and the reputation of the model. If the cars appear to be identical in all aspects except for price, then it would seem economically sensible to choose the car that costs $4,000.
However, a more expensive car might come with added benefits, such as a dealer warranty, recent maintenance updates, or additional features that justify the extra $600. Without more information, Marvin should inspect each car thoroughly, possibly with a mechanic, and negotiate to get the best value for the price.