The Desperately Dirty Lens

I bought a lens on an auction site online. The description was brief; the photograph of the lens was out-of-focus and taken from an angle which didn't reveal much detail. I didn't pay much; most potential bidders were (rightly) suspicious.
When the lens arrived, it lacked lens caps back and front. It looked as though it had been stored in a garden shed for several years. I cleaned off the dirt which revealed a few scratches on the front element and a bit of dust inside the barrel. But the focus motor worked well and it made decent pictures.
You can tolerate a lot of dust and scratches before image quality degrades to unacceptable levels. I use this lens mounted on an equally cheap camera in crowded city streets. If they are damaged or stolen I shan't weep much over their loss.

Canon EOS 30D + Canon EF 35-135mm f/4-5.6 Ultrasonic lens.