Part of the reason you are seeing sellers wanting more than 50% of what MSRP *was* is IMO due to the following:
1. MSRPs have increased due to price increases from vendors. When a vendor increases the price for an item it raises the value for the same used items.
2. Overall inflation in the economy.
3. Most dealers, and I accept that there may be some exceptions, will not give more than 20% off new MSRP. Therefore a discount on a used item somewhere below 20% off will be attractive to someone. Is it 25%? 30% 40 or 50%? Depends on items age condition, the item itself, the buyer and the seller plus other factors.
Its an arms length transaction. Plus buyers are not paying sales tax
4. Sellers have other ways to monetize or get rid of gear. They can give it to family. They can donate it. They can trade it in. Results with all of this can vary. But depending on factors involved it can be an option and may set a floor on what a seller will accept in a purchase.