KILN HOUSE is not the only pre-Rumours era album to sell more than 500,000. FUTURE GAMES, MYSTERY TO ME and BARE TREES are certified gold by the RIAA (for sales in excess of 500,000).
Check out
http://www.riaa.com
SAY YOU WILL is doing extremely well. Why does everyone think it is not?? 770,000 copies is nothing to sneeze at, and it appears very likely that it will be certified platinum by year's end (pending strong Thanksgiving/Christmas shopping sales).
However, VERY BEST OF FM was certified platinum by the RIAA back in January, so wouldn't that mean it sold more than 1 million copies?? (This list is saying it has sales of only 420,000.) But perhaps double albums are counted differently.