In general, you can avoid a lot of headaches by choosing to purchase only from established sellers that have recent and past positive feedback as a seller for selling items in the same category that you intend to buy. If someone is selling a dozen apple watches, but has no feedback, or only has feedback as a buyer, or only has feedback for selling dress patterns or fishing lures (actual examples I have seen), perhaps wait until they have a track record of positive feedback for selling electronics before you buy from them.
You might miss out on a good deal from a new seller once in a while, and nothing can guarantee that you won't still run into an issue anyway, but you can avoid a lot of potential problems by choosing to do business with established sellers. Anyone can put together a listing that looks nice -- it is not hard at all to simply copy an existing listing -- but it is much harder to create a history of consistent sales and positive feedback.
For more experienced sellers, you can learn a lot about the sort of seller you are dealing with by checking the seller's feedback page to see where the seller is registered and to notice if the seller has any negative feedback. If you see negatives indicated in the table of recent feedback, you can choose to exclusively see each type of comment by clicking on the numbers shown in the table. Reading negative or neutral comments can often tell you if a seller has a pattern of problems with items not matching the descriptions, canceled orders, late deliveries or item location misrepresentation.
If a seller has a pattern of too many serious negatives -- or just not enough positive history -- it is often better to find another seller with a better track record to do business with.
If you have made a purchase from a listing that you think may be suspicious:
Typically you have to wait until the last estimated delivery date has passed to start an Item Not Received claim on eBay; if the seller is removed and eBay does not send you a message about that, and if you can no longer find the item in your Purchase History or the Help page, you can start a similar process through your PayPal account (if you used PayPal) or through your credit card or other payment source. There are various deadlines that have to be met at each stage, and you must take care not to make more than one claim about a single transaction through eBay or PayPal or your funding source at a time or your claim may be dismissed.
If the seller actually ships something (but not your item) and can provide a tracking number that shows delivery, you must file a Not As Described claim (and typically return the item), as a valid tracking number that shows delivery would cause you to lose an Item Not Received claim. There are other scams that involve shipping an item to another location in your zip code which can complicate matters further.
If possible bookmark the seller's feedback page so that you can monitor that to see if the seller's account has been removed from eBay, just in case the item listing disappears.
If the seller's listings have all been removed, most likely they were removed by eBay, not the seller.
