Sellers are responsible for the content of their listings; eBay has no way of knowing in advance if the seller has the items on hand, is describing them correctly or intends to ship anything at all.
Sellers are not supposed to list items they do not have on hand (or at least under contract in the case of drop-shippers), but not all sellers are diligent about following the rules.
Even the best of sellers can sometimes lose track of inventory, and when sellers cancel for "out of stock", eBay penalizes them for it. Sellers that have multiple "out of stock" cancellations can wind up losing account privileges quickly.
If a seller tells you an item is out of stock but actually cancels using a different excuse, report the seller to eBay.
https://www.ebay.com/help/action?topicId=4850
You can avoid many issues by choosing only to do business with experienced sellers that have a demonstrated track record of delivering what they advertise, and by avoiding new or inexperienced sellers or sellers that have feedback that indicates a pattern of problems delivering what was promised.
