Hello,
I too agree with you on the issue of SNAD (significantly not as described.)
Product not working is a warranty issue and a snad dispute cannot and should not be opened by the buyer for the same.
eBay attitude towards disputes is totally one sided - favoring the buyer without hearing the seller's side. And if there are too many disputes (or even unfavourable ratings) on the seller's account (however unjustified), eBay will simply suspend the seller for non - performance. Upon which the seller will have to satisfy each and every buyer who has grumbled against him(fairly, unfairly or just to cheat the sellers) to be able to trade on the eBay.in platform again.
However there is a very simple solution in your case. You just have to sign the product which you send with a permanent marker, so you know when the buyer send the product back to you whether it is yours or he has substituted it with another product.