If sales weren't steady HarperCollins wouldn't have made an offer to buy the rights to the top Black Sparrow authors. That being said, they didn't pay a lot, and Ecco is a "perennial" imprint - they put out books that don't sell a lot, but reliably small numbers. You know, small compared to best sellers.
It's interesting though, most best sellers do not sell huge numbers. You can get onto the NYT best seller list with as little as 40,000 sales in a week. That's a lot, but it's a drop in the bucket compared to what big books sell. Or mid-level hip hop groups.
"Long-seller" as used here sounds to me like "a movie with legs." When I managed a movie theater way back when, a film like Crocodile Dundee had legs because it ran forever and was a modest but steady and reliable money-maker. The opposite of "flash-in-the-pan".
according to info from different sources, this applies for the US too.
I'm pretty sure it isn't the case for Germany in general, but I know from the officials at Andernach, that Bukowski-books were regularly stolen from their public library, so they finally had to lock them.