[John Jarrold]

Two mutually exclusive answers, I'm afraid. Series sell far better than stand-alones. But if no one buys the first book, no one will want the second. So you can have an ending that would work for a series if required, but don't put all your eggs in one basket. It's also possible to write books in the same universe that aren't direct sequels - like Iain M Banks' Culture novels - so if the first book doesn't sell, you don't have to scrap an entire series...