I mentioned above that the comics had Parker unsuccessfully trying to sell the fluid, but the reason why he can't maybe isn't that logical. Maybe there are applications that a temporary dissolving glue would be ideal for, like emergency surgery or as mentioned, non-lethal web-guns for the police.

And maybe it's more logical that a bite from a radioactive (or genetically altered) spider would either have no effect, or would kill him outright. And maybe it's more logical that a costume a solitary high school student designs would not look like something out of Nike's 21st century design and development team for olympic athletes.

Or maybe what's MOST logical is to go with the concepts that Stan and Steve introduced, which have worked and endured for decades.

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But that would deny Peter David his God given right to poo-poo the work of others, and show us all how to do it RIGHT!!

Frankly, with all the "reality" in comics today, I'm amazed that Spider-Man hasn't been stalked by the EPA concerning the potential toxic chemicals in his web fluid and made to answer for the environmental harm his leftover webs have possibly caused. All we need are a couple of street thugs to claim they have "Spider-Web Syndrome" from being webbed up in the midst of their crime, and we've got a major lawsuit and a health hazard scare...

And let's not forget Homeland Security and the NYPD launching an investigation in why this high school kid is repeatedly purchasing all these various chemicals for unknown purposes...

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Other than it being a really, really, really dumb idea -- and one the CREATORS of the character rejected?

Reasons the organic webbing is dumb:

1) Where does it come from? Spiders do not shoot webbing from the tips of their legs. In fact, they don't SHOOT webbing at all. If Spider-Man has organic webbing, the first thing that needs to be explained is why it doesn't emerge from a spinerette at the base of his spine, and why it emerges under high pressure.

2) Where does he store it? Spider's produce the webbing they need as they need it. This is strands more slender than a human hair, produced slowly. Spider-Man squirts out webbing much thicker than a hair (as drawn by Ditko), and much thicker than most spaghetti (as drawn by most people in the past couple of decades). Seems unlikely he could produce that much webbing as fast as he'd need it, so he would need to store it. Assuming the web spinners are, for some illogical reason, located at the ends of his arms, wouldn't this leave him looking like Popeye?

3) How much strain does it put on him producing this webbing? Take a look at any scene in the movies. He swings thru the city on threads -- THICK threads -- hundreds of feet long. What's being used to make this stuff? Is he channelling it from the "web dimension"? Wouldn't churning out that much webbing leave him, at the very least, dehydrated? And at the very worst, pretty much an empty husk?

4) What happens when he runs out? Given the above, he clearly cannot possibly have an inexhaustible supply. Even if he is storing it in those Popeye arms, one good tussle with Doctor Octopus or the Sandman, and he's going to be taxing his resources. Spider-Man with mechanical webspinners occasionally runs out, and there is a dramatic moment as he switches cartridges. Spider-Man with organic webspinners runs out and there is a dramatic moment as he FALLS TO HIS DEATH.

These are almost certainly things Stan and Steve considered. Why else go for the mechanical webshooters? Stan has even said he CONSIDERED organic webspinners -- but didn't go that way.

If it ain't broke. . .

"Here's my problem - when you start picking at the conventions of superhero comics, the entire world becomes undone.  "Why doesn't Peter Parker make money off his web fluid?" "

For comics, I agree it is a convention we just let go. Film is a different matter. It would be easy to say he is a genius and can invent whatever. It would additonally be easy to say he doesn't sell it because is concern is being heroic. In a film, those conventions could be acceptable. But there is a third mater, presented quite strong in the film, that he SHOULD sell it if he could. That would be Aunt May.

Peter might not care about the money his genius can make. But he demonstrated his care for financially supporting Aunt May, someone who gives money to Peter that he knowsshe can't afford. If he had the ability to provide for better financial support, and refused to do so, he would just be a selfish dick.