The UTDA (Used Tractor Dealors Association) member list is a good place to look for responsible dealers. I purchased from one of the members 6 years ago, great tractor.
UTDA Membership List
I completely agree with picking a dealer from this list. The Yanmars are great tractors (my 790 is one branded by JD) but if they need to be refurbished I would run away. It is inconceivable, given the price for parts, that a tractor could really be economically "rebuilt". The good dealers pick the best of the grey market tractors and may fix a thing or two. The rest get "refurbished" and many show up as sad stories on TBN. I had one and was very lucky that my dealer took it back.