I am following with great interest a listing on ebay for a used Guarneri pattern Gama that I sold to a customer in 2012 and which he has now replaced with an instrument costing about 4k.
This violin was first listed at £550 (exactly the same price as I list my new Gamas on ebay - yes it is more expensive than on my own website, it's the fees) which to be fair does include a case. Then it gradually went down to £500, reducing by £10 a time. Then up to £599, then up to £625, then £635, then £665.
I have heard tales before that listing items with a higher price can have the effect of making people think they are higher quality (people have suggested the same thing to me with my kittens). It will be very interesting indeed to see what price this instrument sells at.
As to whether it is worth it, well yes. If you went into a respected bricks and mortar violin shop would would certainly be paying more than that for it, even used. And yes they do improve with playing. But in my experience people expect a significant discount just for the fact of it being used (unless of course they are trading in, in which case they expect to get nearly all their money back. That doesn't work as a business model!)
If anyone buys it, do let me know.