Some thoughts:
1. Technology and continual modifications continue jumping forward, rendering previous models dramatically less desirable, much harder to sell, and a price that falls accordingly. Including non-substantial "improvements." It's hard to know in advance what's important or not and it affects the resale market strongly.
2. These are sensitive instruments and can get "used up" with the salt, sun, knocking around, etc, they are subjected to. I think we don't want to buy someone else's problems when they feel it's time for an upgrade themselves.
3. In the case of sensitive and changing technology like this, I think the manufactures warranty is important. If it craps out, they replace it. Ebay doesn't work like that.
4. Consider the difference in price carefully between smart shopping and buying new, and the exposure to many risks you will have by buying one that's unknown, used, and not under warranty. Your loss will potentially be the amount you pay, not the amount you save. If you are paying $50 for an expensive and relatively new fish finder, then it's $50 that you stand to lose. That might be a good risk. If you are saving $50 over the price of a new one under warranty, then you might be penny wise and pound foolish.
5. Knowing the history of the fish finder yourself, and the person selling it is another consideration.
Good luck
