I was looking at the classifieds on the hull truth a couple of days ago, there was a post from a guy selling several different APs that had been removed. I don't have relation to the seller just saw the listing... might be of interest.
I have no problem buying used radars, plotters, radios or depthsounder. They can be tested before purchase or right after delivery during the return period on eBay.
I don't recommend buying a used autopilot unless you know what you are doing or it comes with a 10 - 20 day guarantee from a reputable source. Too many components to test, too many things to not work and difficult and time consuming to test.
I buy and sell used marine equipment on eBay and CL. Whenever I get an used autopilot, I sell the parts as "parts or not working" on eBay. There are people on eBay that buy all the parts to an autopilot whether working or not and either repair the broken component or gather enough working parts to get a working complete AP. They sell the working AP on eBay, who guarantee the items and provides protection for the buyer.
The buyer has to either bench test the AP or install it quickly before the guarantee runs out.
I've sold several Furuno 700 series AP's and they are very good AP's. The control unit, processor, rudder feedback and heading sensor is around $3,000. The pump is not included. Furuno supports their legacy products for a longer period than any other manufacturer.
The Simrad AP's are excellent also.
I would not purchase Raymarine because of their questionable future, their lack of support for legacy products and their past history of ownership changes.
Garmin does not support legacy products either.