First of all you have to search for a reputed dealer. Then ask him all necessary questions like what sort of lenses shall fit in your camera, what shall be the size of the lens to fit in your camera. If you are going to purchase it online then you have to ask whether the surface of the lens is scratched and marked in any way. Make sure that three is no mould or scratches on the lenses. You can also hold the lens towards the sun or a light source. Do not hold it directly against the sun. Now you have to inspect the surface of the lens. If you find any dark marks that shall indicate moulds. So then you have to clean the lens using a cleaning brush. Also you have to check whether the focusing lens when switched to manual settings is working fine. There should be free movement of the focusing gear. If you are buying a zoom lens then it should be able to zoom in and out freely. Also check whether the outside of the lens is in good condition. Then check the contacts at the back of the lens which must be in good condition and should not be worn out or broken. At the other end you have to unscrew the filter and check the lens for any scratch marks. Also ask whether they have a return policy because reputed dealers offer return service but you have to pay for the postage. Also get the insurance for the postal damage. That‚Äôs it.