eBay's Money Back Guarantee system is - in a nutshell and pretty much exclusively - a refund request system, and once you treat it as that, you may find you get the resolution you want more often than not.
I understand that there are sellers using tactics to string request out and so on, but I'm speaking a little outside that context at the moment - the request system is not set up for replacments. As a seller I would love it if it was, because when a problem arises, that's what most of my buyers want, but as soon as a request is open, that option is off the table. If they contact me without opening a request, they can have whatever option they want. So I would suggest contacting sellers directly, waiting for a resolution, and if they send replacements, keep an eye on the deadlines and open the eBay or PayPal dispute if / when needed,
My ideal system would allow for the buyer to accept a replacement and the seller to send it, with the uploading of a new, verified as valid (this bit would be important, as it would hopefully prevent sellers from uploading fake numbers to trigger an extension), tracking number extending the time for the case to be able to be escalated in order to allow for its arrival (I always send replacements with full tracking no matter what the original price and postage method was, but the seller should retain the option to refund instead, for obvious reasons).
