What is the percentage at which an auction sale is considered permissible and not an undervaluation against the debtor, taking into account that auctions often sell for less than the appraised value, and is it 50%, or 70%, or more?

If the debtor's wealth is insufficient to repay his debt, or if he mortgaged something and did not repay his debt, then his belongings are sold to repay the debt, provided that they are sold at a fair market price (price of similar goods) or higher. It is not permissible to sell at a significantly lower price unless the reduction is minor and commonly tolerated by people. If markets are stagnant and prices have dropped excessively, the belongings are usually not sold until prices rise, unless the belongings are susceptible to damage or there is no hope of prices rising. If the belongings are sold at a significant discount from the fair market price without necessity, it appears that purchasing them is not permissible.