Recently I purchase two used prime lenses for my Nikon D750 on eBay. One is an AF Nikkor 50 mm f1.4 D lens that I am using for street photography and the other an AF Micro Nikkor 105 mm f2.8 D. Before purchasing I did my research and found both lenses to be excellent choices. They perform competitively with the newer Nikon models. I have been happy with the results.
This approach would probably work with other brands of cameras and other types of cameras, such as micro four thirds. For example, I was looking to buy a Lumix DMC-GF1 camera body. There were a number of great used Olympus lenses available for the camera at a great price that would have met my needs. To make this approach work, you need to research older lens series for your particular camera. There are many lens reviews available on the Internet.
When buying from eBay I did not go for the lowest price. I carefully studied the photos of the lenses for wear or indications of misuse or abuse. In both cases I used the “Buy It Now” option because I didn’t want to end up in a bidding battle and I found the prices comparable to the final auction prices anyway. I steered away from lenses where the photos of the lenses were poor or where there were not enough different views to be able to see all sides. The descriptions sometimes turned me away from some sellers because they used language that made it appear that they did not know what they were selling.
Depending on the seller, I was also happy to read that there was no apparent dust, fungus, or oil inside the lens and that there were no scratches or other imperfections on the glass. The condition of the optics is something that you cannot determine from photos. That kind of detail was less important when I was looking at lenses from a dealer that specialized in lenses.
In addition to the precautions above, you should be certain that the used lens is compatible with your camera body. In some cases you may lose the use of some camera features, but the lens will still function. At the extreme, I know that many older Nikon lenses will fit my camera, but will not auto-focus. Some of the third party lenses will support focus verification in the viewfinder although they are manual focus lenses. There are all kinds of variations and you may find that you can justify the limitations for a specific lens. In the case of the prime lenses that I bought, all camera features are supported.
I paid about half the price that I would have paid for a new lens of the latest build and I received lenses that perform as well. No matter what you use for a camera body, I am convinced that you can do well with used lenses. Shop around and be critical to get the best value.