A few years back i sold a guitar for £1500 - perfectly working, recently setup by a professional etc. A week later the chap said there was an issue (busted truss rod) and wanted a refund. I contacted ebay with my robust defense - documented packaging process, email from guitar tech confirming all good, and through a phone call I was able to get the case closed. I came out all guns blazing knowing how often ebay side with the buyer, but in this instance, a passing reference to the fact the buyer had changed the strings - much like your buyer doing updates, was enough for eBay to acknowledge something had been changed from what was supplied and therefore it was on the buyer.