The best place that I've found to buy a
second-hand Jura machine at a reasonable price is on eBay Germany.
Many of the machines are fully refurbished (cleaned, seals replaced, etc) and sold with a 12-month guarantee. They range from around EUR250 for a machine with a single boiler (perfect if you drink mainly espresso) to around EUR350 for machine with double boilers (a better choice if you make lots of milk-based drinks, as you don't have to wait for the system to heat/cool when changing between 'coffee' and 'steam' settings)
I've bought a couple of Jura machines from eBay Germany and all were still going strong at the 5-year mark. They weren't 'factory fresh' but they were awfully close, and at about 20% of what you'd pay for a new one.