Post-auction buy (or post-auction sale, or "after sale" in French) is a term used for sales of lots not sold at a particular auction, either because there was no bid or because the highest bid did not meet the reserve. As soon as the auction ends, you can make an offer to the auction house if you are interested in buying an artwork at post-auction sale. Please note that, in this case, the same fees apply to the post-auction sale as if it was sold during the auction.