H.C. Roeters van Lennep was one of the most famous Dutch shell collectors during the mid-19th century, for whom we here present new and additional biographical information. His collection was auctioned in 1876, but so far only a limited amount of information has been published on this topic. The details of the auction are reconstructed on the basis of his correspondence with H. Crosse. Such new information provides an insight into who buyers were during the auction, which prices were realised, and where parts of the material ended up. There appears to have been a second auction in 1879 where possibly a large part of the remainder of the first auction was sold.