I know Kadano has bought a bunch of used ones off ebay which I happened to test and they were all quite good. He generally recommends Japanese sellers as their condition labels and pictures are usually very accurate and reliable. I think with Hori Controllers you're only able to find Japanese sellers anyways..
I would recommend just buying one that has high quality pictures from which you can tell the condition of the controller and ask the seller whether the item being sold is the one in the picture. Apart from that I would pick a reliable seller (1000+ feedback, 98%+ positive)
Regarding the controller, I liked the shape of the controller and of the buttons but didn't like the way the buttons felt but I think you can definitely get used to it.