Sometimes I admit it is frustrating to see used units priced too high. It's not uncommon for me to see used items sold on ebay for more than I can buy them for new somewhere else online.
It's also hard for the seller to take such a huge loss usually. If you think about it, a customer locally here (no one in town has the "x" units yet) goes and buys a 76cs for $500, not knowning that he can buy them online for nearly $350, so he decides to sell it in 2 months what is a fair used price $250 at best.... it's hard to swallow a 50% loss in 8 weeks. Digital cameras are the same way, the only plus is that while those things drop very quickly prices of the new units do as well. For example I sold a 5mp digital camera that cost me $800 in 2002 for $250, a huge loss, but I was able to buy a new 5mp camera with more zoom, vibration reduction, better image quality and performance for $400, so while I lost a lot of resale value, it only cost me $150 to upgrade to a much better unit.
But I think in the end as others have said it's what the market will bear, I think the best sellers will try to name a fair price, but again everyone has a different idea of fair. Some sellers don't know what you can buy them for online as well. You can already order a 76csx for about $450 online if you look around. It also depends on if the seller is in a hurry, or motivated to sell, condition of the unit, accessories etc. It's just as frustrating to sellers I'm sure to list something at what you feel is a fair price, say $200 and get bombarded with emails asking if you will sell the unit for $75.
Dealers are no better, for instance I've asked a few places in town about the new garmin "x" units that use memory cards and every sales guy I've asked has said they have not heard anything about a new unit coming out(I know this isn't true because a buddy works at one of the stores and gets product updates and has been aware of it for some time). So is it really any better for a store to try and mislead a customer to buy a 76cs for their $500 "on sale" price? Another store's hunting dept. manager who's corporate website specifically says their stores will price match internet sites, flat out told me they have never done that and would not do it. After a letter to the corporate office, the store manager assures me that "mistake" will not happen again.
If you think gps units are bad, try camera lenses. This is one that really blows my mind. High end camera lenses used without warranty, as a mater of rule not exception get 90% of their new cost, if not more. Why people pay $1000 for a used lens with no warranty they can buy one new with warranty from a dealer for $1150 I have no idea, but it happens all the time.
I think in the end, good/great deals will come along if you are patient and willing to shop around. As I'm sure is true of sellers, if you are patient you can probably get a higher price for your gps it just may take longer to sell.