There's a very slim chance nowadays that a publisher will take on an entire series or contract more books from the start.
This may be genre specific, and I realize it's also anecdotal, but everyone I know personally who sold a first fantasy novel to a medium to large publisher got a contract for at least two books initially, and trilogies, even longer series aren't unknown (though the latter can have kill clauses if sales aren't where they "should" be by a certain point--this happened to one friend's contracted multi-book series). Completely alone (stand alone with no sequel) fantasy novels from unknowns aren't terribly marketable, and I've heard that it usually takes 2-3 books to ramp up sales into something that's profitable for the publisher.
But that doesn't mean the first book shouldn't come to a satisfactory, self-contained conclusion, even if it's very open for a sequel or series to follow.
And it's possibly a good idea to have drafts of the second novel or two on hand, at least, when you query the first. I know someone who sold her first novel (a paranormal romance) recently and got a three-book deal, but they won't bring out the first one until the second and third ones are in the publication pipeline at least, and she's on a brutal deadline. The explanation given her is that the publishers don't want more than a 3 month delay between book one and book two in a series for new authors. Subsequent books boost sales of the first one, and readers tend to lose interest in and forget about new authors if the first book comes out followed by a long wait for book 2.
Small publishers (or publishers outside SFF) may have a different marketing strategy, of course, and if you're already established and selling well as an author, longer delays between installments are more likely to be tolerated.
But I'd agree that sequels to a novel that is already self published would be a hard sell. The word needs to get out there that agents and editors are
not generally looking to pick up books that are already self published unless sales are exceptionally strong and persistent. I'd definitely make "Book 2" in your series read like it's "Book 1" to a new series, not a sequel to your self-published book, even if it's about the same characters.