So - what work does he say needs doing? And is this a fair statement or not. How much was he quoted for the work? Does it seem a fair quote? Who did the quote? And of course, how did you describe the sax? Have you got proof of the condition it was in when he took it? Was the sax too expensive due to bidding fever?
If he's got fair grounds for his complaint, then best to take it back. If, however he's making it up, then stand your ground and ask him to go through dispute resolution on ebay. But be aware he may also just be trying to get the price down, not give the sax back. Or he may have damaged it and is trying to get out without telling you. Whether or not he collects shouldn't affect buyer protection - but it does mean you have a better argument that he's changed his mind/is time wasting, than if you'd posted it.
The vast majority of people on ebay are great to deal with, but there are a few others who seem to be around just to remind us that it's not all plain sailing...