Are there penalties to posting goods you don't have and selling them before you have them in-hand? Does Amazon penalize sellers for cancelling orders if they can't be delivered?
That's called drop shipping when you list an item you don't have. There's a healthy community of drop shippers in most e-commerce sites.
I'm sure Amazon and other sites do penalize for cancelling orders, so one has to do a cost-benefit analysis if they make a mistake in pricing. E.g. the source increased prices recently but do you want to risk your reputation of your online store?
The risk you're taking on is that you can fulfill the order more cheaply than what the customer bought it for.
Amazon has performance targets that sellers should meet. Last time I saw them, the Pre-fulfillment Cancel Rate target is under 2.5% and the Late Shipment Rate is under 4%. (I'm not sure if they're different for different types of sellers on Amazon or if those are blanket targets for everyone, though.) If you end up on the wrong side of those targets, Amazon sends a warning, but you're not immediately suspended. It's unclear to me exactly at what point you do get suspended... I've seen it happen only when one of those rates gets up to 10% or so, but I've also seen it happen when one of those rates just goes about 1% higher than the target, so I suspect there are a lot of factors behind the scene that are considered (by an algorithm) before suspension/loss of selling privileges like negative feedback rates and other data Amazon measures for each seller.
I'm not a dropshipper, btw. Just someone who's been in charge of a book store's online sales (including on Amazon) for a while. Dropshippers actually drive me crazy because they're usually really bad customers. Many of them make lots of demands of the seller like emailing the tracking number separately (even though Amazon emails it automatically), and removing any pricing info from packing slips, or removing my store's name from packing slips. One drop shipper even asked that I put their store's name onto our packing slip and remove my store's name. All these demands that dropshippers have really slow down our order processing, since almost every single other order we get has no special demands.
Additionally, since they're not the end recipient of the shipments, if the end recipient has a question or issue they have to pass it along to us, and they often do so very poorly. They're often rude, frequently muddle the question, and sometimes make additional, outlandish demands, like "my customer thinks it's taking too long. send another book via overnight shipping or I'll leave a negative review!" I try to report as many of the worst offenders as possible to Amazon, but that takes time out of what I should be working on instead, and often Amazon doesn't care because Amazon Support can be almost as bad.
Amazon does keep track of many seller performance metrics, including cancellation rate. I don't know what the softer penalties are, but they can and do freeze accounts, which is obviously horrible if it is your primary income.
They don't care whether you actually have the item in hand as long as you ship it in a timely fashion.
