Over the years I have bought and sold many cameras and lenses. The key is to be competitive since, as they say, the value of something is what people pay for it.
For a camera or lens that is current and is still being sold new, I generally discount it 20-30% from the lowest retail price, excluding taxes. This means that buyers save up to 25% in Quebec where federal plus provincial taxes add up to around 15%.
For one which is not current, you have to establish the value of an equivalent camera. Something with the same sensor size and zoom, in case of a fixed lens camera. Then discount 20-30% from that.
Another control-point for setting sale price is to check what the same model is being sold for by other people. Some models have a better reputation and so get less of a discount compared to the current price, others more, specially if there was a major leap in technology since then.
Lens keep their value well and high-end ones can even be sold for more than purchased since many manufacturers raise their prices over time, until a replacement lens is announced at least. Again, what you have to aim for is that the buyer gets a nice discount for buying something that is out of warranty.