I don't agree with khun ayethebing about the interpretation.
After considering this story, i think that it would like to remind the investors to buy a good stock rather than a bad one. However, investors can make a mistake when selecting a stock, so once investors realize about it, they should not keep that bad stock anymore.
About selling at 10% or 20% discounted, it would imply that investors should not wait to sell it out when they know that it is bad stock. The longer you keep it, the more investors will lose coz it might have no value anymore in the future.