There are a number of reasons other than just being used that a used bow will sell at a substantial discount from a new bow.
If you buy a new bow from a recognized bowyer, he will make good on any defects that turn up within a reasonable amount of time after you buy the bow. Even if it isn't actually a defect, but just that the bow didn't perform as expected, my experience is that a bowyer will try to make a buyer happy. Even if a problem is admittedly caused by the buyer, a bowyer will do what he can to help ease the pain. For example, a number of years ago I had a bow dry fire because the arrow nock was too loose and the arrow fell off the string at full draw. The shock caused the bow to fly out of my bow hand and the riser cracked when it hit the concrete patio I was standing on. I called the bowyer and he sold me another riser he happened to have laying around for $100.
Very few people sell bows that they really like. If it's a quality bow, maybe the buyer will like it better than the seller did, or maybe the seller got old and couldn't pull the bow anymore, or just bought a bow that was too heavy to start with, or maybe he fell into hard times and just needed the money. But the fact is there are bows out there that have been sold and resold a number of times and are probably just dogs. When a bowyer makes something out of wood, no matter what his reputation, he can't guarantee that it will always turn out like he wanted it to.
Buyers and sellers of used bows often disagree on what is a cosmetic blemish and what is a structural defect. Most sellers of used bows here on Tradgang would be willing to take a bow back if the buyer doesn't like it and returns it right away. However, that is still a hassle and there is the question of return shipping charges.
All that said, I have been very happy with my experience buying and selling bows here on TradGang. It has given me the chance to try a number of bows I wouldn't be able to try otherwise at a very low cost, as once the big new/used discount has been taken, you really shouldn't lose much money on buying a used bow if you know what you're doing.
In the end, it comes down to supply and demand, like everything else. A bow may be "worth" $1,000 to a seller, but if nobody else is willing to pay that, then he's not going to be able to sell that bow for $1,000.