There are many reasons why the price is removed when the item is sold.
One is that the deal is between the seller and the buyer.
One more important aspect is to keep the prices high for used gear. If you are going to buy used you naturally want to see what that item usually sells for. If you then see for exemple that a person has sold GS1000 for $600, then you might not want to buy GS1000 for $700, which might still be a good price.
If you want to know what items sell for then look at the asking prices in the for sell forums. If you keep looking you will see a trend.
It's better to keep the used prices high, so that the people who buys the item in the first place doesn't lose that much money when he sells it. Otherwise people would not like to buy new stuff since they don't get much money when the sell it, and then the used market would probably be much more scarce.
EDIT: As it is now, people can buy new gear, then if they don't like it they can sell it for 75% of what they bought it from. It's a win win situation for both seller and buyer of used gear.
The alternative is that people only buys new gear which they know they are going to keep, so they don't risk selling it for 30%-50% of what they bough it for. This would reduce the amount of gear avaliable to the buyer of used gear.
