The point of the article is the answer to this: they were systematically overpaying because the offers where they overestimate value were disproportionately taken up by sellers.

But also there's the cost of inspection, property insurance, title insurance, escrow fees, labour costs of making the sale etc. As the article says, trading houses is not like trading stocks.

They pretty much lay it out in the article. Essentially they end up with the "lemons" and then have to pay to hold them until they sell.

The only offers they made that were accepted were the ones on shitty houses that their algorithm mispriced.