Qitzur Shulchan Arukh 62:3
Someone who buys and sells with trustworthiness never has to worry about cheating [anyone]. How so? [Let him say,] “This property I bought for this and this [price], and I wish to sell it for that and that”. Even if he was cheated when he bought it, and one who was cheated doesn’t have permission to cheat others because of it, in any case it’s allowed. Because he explained to [the buyer] not to depend on the value of the item, but rather on what he paid for it.
In the previous installment, we noted that halachic business deal requires one charge what the item is worth. This includes a risk, since if the seller over-estimates the worth, he will accidentally violate the prohibition of ona’ah. Therefore the Qitzur recommends being up front about one’s own purchase price. By explicitly tying the deal to purchase price rather than a straight buying of an object of a particular value, the deal is valid regardless of the price.