Originally Posted by Retro Grouch
With a used bike, however, I'm betting that you have a hosed crankarm. When the bolt that holds the crankarm is loose, it allows the crankarm to wallow around the square spindle and tries to turn that square hole into a round one. The upshot is that the crankarm will never stay tight after that.
The good news is that left crankarms are available singly as repair parts and are usually not too expensive. Any average or better LBS will be able to get one for you, especially if you aren't too picky about having an exact replacement.
Yeah, what he said. Find a good bike shop, preferably one with a pile of old bikes in the basement. When this happened to me, the dude went downstairs & swapped the crank for me. Wouldn't let me pay him either. Of course it doesn't match the right side, but who cares? The bike is 12 years old & the new arm didn't cost me.