You've probably run across an item from a person who sells items from a drop shipping company or large supplier. The person lists items for sale on their ebay account. When they get an order, they place the order with the company. The company goes to ship the item and find it's out of stock. They let the seller know, and of course the seller has to cancel with the buyer.
The reason for listing with ridiculous prices: Large sellers do that if their item is out of stock. In order to prevent/discourage people from buying the item, they hike the price to a ridiculous level and leave the listing where it is. This is called a placeholder listing. It is a policy violation for the seller to do that, but people do it anyway. When the item is back in stock, they return the price to it's original.
Book sellers on Amazon are notorious for this...so when you see a copy of your favorite adult coloring book selling for $995.00, that's not the real price...they're just out of stock Smiley Wink