My mother is also a writer, and I’m her publisher. It’s a bit of a weird relationship to have with your parent, but hey, there it is.
She’s (obviously) from an older generation, one that has different ideas about certain things. One of those ideas is having an inventory of books. She wants to have boxes of books, so she can go places and give them large numbers of books to sell? I really don’t know the motivation behind it. To me it seems like a great way to have a stairwell covered in books.
I do print on demand. If I need a copy of a book I buy it, then I sell it. The cost is the same if I buy one or a hundred books. I can still do a large order, if I need a hundred books I order a hundred books…
Suppose I want to do a convention (which I am thinking about doing as time goes on), I can order all of my books a few weeks in advance, and hit the con with a table full of novels. No issue with that. On the flip side, I don’t have a space filled with books, and the tax man doesn’t view my house full of books as a material asset that I can be taxed on (not sure if that applies to writers, it does for musicians with audio recordings – I have had friends and family screwed by this).
Of course if you order enough of an inventory you get to enjoy a fairly significant discount, but the numbers you need are in the thousands (five hundred gets you almost no discount, it really starts to get decent when you get to a thousand copies). So, if you are cool with having 5k books in your house (and possibly paying tax on them), and you have the money to buy all of them up front, great, you can save some bucks on your books.
Me, I’m using POD for all of my books. I don’t mind the rates they are charging me. It’s low enough that I still make a few bucks per copy, and my cash flow isn’t destroyed by it.