Depends a lot on what you are looking for and what you want to spend. Like Slabprowler said, about the only thing if you don't know the unit is HR's. Less than 500 is not much use. Cracked screen, scuffed up exterior tells you someone didn't take very good care of it. Like Lowe175 said, if your going to spend 300 or more you can just about buy a brand new unit as cheap or cheaper than a good used unit today. I watch bass boat central several times a day just to see what stuff is selling for. Guys will have 3-4 year old units priced higher than you can buy a comparable new unit for yet some guys buy them. Makes no sense to me.