If it's something I think I'm going to reread, sure. If it's something to complete a collection where I already own volumes, maybe.

I read a lot of books from my local library. But like all underfunded local libraries, the selection is rather hit and miss; there are quite a few examples of series where they don't have the complete set. If I continue the series past what they have, I need to buy them. On a few occasions, I've gone back to buy the ones that I've already read just because it pains me to own a partial series.

But in general, I buy fewer books than I used to. Partly because money isn't as free and easy as it used to be, partly because my house is already full of books and I can't just keep buying them until I'm buried alive.