Like anything else, photography attracts a good amount of resales of camera equipment. People buy lenses for instance, only to discover they do not fit the way they shoot. Other people upgrade, and decide to sell their current equipment.
So how much are used goods worth?
When it comes to cameras, be very careful. If you have noticed the amount of new technology coming out over the past 12 months, you should be very wary about buying anything second hand.
Check first that the camera is the current model, and when the upgrade is likely. Then you can use some basic maths to get the true value figure.
Brand new: The current average new price, less the tax, less 10%. The original buyer paid a percentage of the price to the government as tax. You should not repeat their mistake.
Very good: 65% of new price, less the tax.
Good: 40% of the new price, less the tax.
Less than good, steer clear all together.
Lenses and other accessories
Pristine: 75% of the price of the cheapest online available.
Very good: 60% of the price of the cheapest online available.
Good: 35% of the price of the cheapest online available.
Just remember, it is always a BUYER's market. Things are only ever worth what someone is willing to pay for them.