The shops which have a good collection of secondhand stuff are mostly at the excelsior shopping centre basement. That's next to peninsular shopping centre, in case you're not familiar with the area. Now that so many guitar shops have congregated there, it makes it good for us to browse around and compare easily.
Prices depend on model, age, and condition... better go with someone who might be more familiar with various guitar brands and models, cos they sometimes have alot, especially Guitar Connection.