Camera lenses are expensive, but you can often pick up a second-hand one cheaply. There's one area you need to be really careful about: inspect the mount thoroughly and walk away if there's a problem.
DIY Photography advises always checking the mount carefully. Is it clean? Are there any scratches? Is it bent or seemingly out of alignment in any way? Are the contacts clean and in good condition?
Remember that this point of connection is the only thing that lets the camera communicate with the lens. This is where it can all go wrong if you're not careful. Other imperfections might not necessarily be deal-breakers, but any and all problems with the mount should be. Just walk away.
The full post at DIY Photography has some great tips, and several common sense ones that overlap with areas like buying a used smartphone. To decide which lens you need and compare options, LensHero is still one of the best options out there.
Best Tips For Buying Used Lenses [DIY Photography]