I'm not a financial...well, anything, but it's my interpretation that most large stock market drops are due to massive selloffs. Basically, people who own the stock are trying to unload it before something bad happens to those stocks. The result is something bad happens to those stocks, and things start crashing.

Like in Sneakers:

People think a bank is financially shaky

Consequence: people start to withdraw their money

Result: Pretty soon it is financially shaky

In other words, stores running out of things and/or online sales being strong aren't, by themselves, the cause of major market movements right now; it's speculation about what'll happen tomorrow, and making sure you don't own any of the stocks if & when it tanks. Which incidentally causes the stocks to tank.

Don't even ask the question. The answer is yes, it's priced in. Think Amazon will beat the next earnings? That's already been priced in. You work at the drive thru for Mickey D's and found out that the burgers are made of human meat? Priced in. You think insiders don't already know that? The market is an all powerful, all encompassing being that knows the very inner workings of your subconscious before you were even born. Your very existence was priced in decades ago when the market was valuing Standard Oil's expected future earnings based on population growth that would lead to your birth, what age you would get a car, how many times you would drive your car every week, how many times you take the bus/train, etc. Anything you can think of has already been priced in, even the things you aren't thinking of. You have no original thoughts. Your consciousness is just an illusion, a product of the omniscent market. Free will is a myth. The market sees all, knows all and will be there from the beginning of time until the end of the universe (the market has already priced in the heat death of the universe). So please, before you make a post somewhere asking whether AAPL has priced in earpods 11 sales or whatever, know that it has already been priced in and don't ask such a dumb fucking question again.

Even online stores are running out of things.

Amazon for instance started running out of generic Tylenol and other items you're recommended to have on hand yesterday.

I was putting stuff in my cart Saturday only to have it be unavailable for the forseeable future by Sunday.

Online merely lags behind brick and mortars. They'll probably have more stuff available since you aren't browsing but searching for specific things, but for things everyone thinks they need, they won't be doing much better.