There's "obvious" bad ideas like trying to sell sand in the desert.
There's the bad (underwhelming) ideas, maybe that's the most common of them, it's probably the 'trying to make a weird idea sellable'. Like 'french fries are amazing but unhealthy so let's try to sell potatoes cut as fries, but baked, with no salt'
There's also quirky ideas as bad ideas, which is the way most people go and what PG probably meant. Not to mention sometimes your idea looks like the same as a failed one already but the devil is in the details.
Depends on your investment strategy. Fads are nearly impossible to predict. So you have $100,000 to invest and this guy comes up to you with pet rocks. Another guy with toilets that sing. Another guy with shoes that play music.
Who do you give the money to? Any of these guys?
Just because fads exist, doesn't mean bad ideas don't. Banking on the pet rock was a huge gamble for someone and it somehow worked out. Fad toys must have 1 out of 100,000 success rate, if that.
I'm skeptical of trying to segment ideas into categories in the first place. It's one thing to say something is a good or bad idea as a rhetorical point, but when we're talking about sets of ideas I think it borders on useless.
At the same time, I have difficulty thinking of any examples of anyone trying to do such an obviously bad idea. The closest analogy that comes to mind for me is trying to penetrate an overly saturated market. Why would anyone want to do that? But hey, you could say that fast food burgers is extremely saturated, and yet, new companies like Five Guys are coming out, surprising, and thriving. Sometimes you can't know until it either succeeds or fails.
Selling sand in the desert isn't _that_ of an obviously bad idea. I bet some people are making good money selling sand for golf bunkers or white beaches in the likes of Abu Dhabi. Even the sand for artificial islands sometimes gets imported from elsewhere because the locally available one is too scarce or lacks some essential qualities.
You got there first. Sand for artificial beaches gets imported all the time, from many far flung places. Desert sand != coral reef sand (the white sand from dead coral, generally excreted from Parrot fish).
You probably do quite nicely selling white sands to the beach resorts of the middle east.
