I usually have very good experiences when buying on eBay. But when I have an issue, it's often related to a seller who does not hold the physical inventory and uses a drop-shipper. The seller creates listings for hundreds or thousands of products and then relies on the drop-shipper for fulfillment, which can be dodgy.
One "tell" if a seller is drop-shipping is to ask them a question about the product they are listing. For example, "What is the expiration date on the packaging?". If you don't get any reply, it's likely because they don't have the physical product.