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 a scammer 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.
It is always better to avoid giving your money to a scammer in the first place, rather than have to go to great lengths to get it back.
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 Resolution Center, you can start a similar process through your PayPal account (if you used PayPal) or your credit card account. 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 at a time or your claim may be dismissed. Generally you are better off trying to go through eBay first, then PayPal, then your payment source.
If the seller actually ships something (but not your item) and can provide a tracking number that shows delivery, you must file a Significantly 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.
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.
