The basic problem is solveable without blockchain by simply registering products by their serial number on the brand's website, which a lot of brands actually already do for higher priced products.
Sure, some people will input fake serial numbers to get a free product, but they probably weighed the cost of counterfeit strings affecting brand image (if I don’t know mine are counterfeit, I’ll assume it’s D’Addario’s fault if they’re bad) with the cost of sending a free set of $5 strings.
You buy them wholesale from the manufacturer?
This has been a solved problem for any legitimate business. The only other place I've seen such lack of quality control were at flea markets, which doesn't say anything good about Amazon.
The problem is that manufacturers need to opt into it and there is no way as a consumer to tell on a product listing whether or not the product is part of the registry.
How can I know for sure though? How does Amazon know for sure? Since that experience I have looked at Amazon in a different light, and I've realized how difficult it is to source name brand products with confidence.
This is as good a time for blockchain as any. Register genuine products on a blockchain and have them scanned once they hit the Amazon warehouse to ensure they are authentic.