I think it's fairly common practice, either when going out of business or just culling the records. I have been offered both the negatives from my first wedding (the photographer was culling his records - offered us the negatives before they were destroyed) and from Ben's senior picture sessions (photographer retired). The main reason to buy them is that without them you'll never again be able to produce a full-resolution photograph from that session. If you had a reason to believe you might one day want new copies of the pictures it might be tempting to have the masters.