I suppose I could say there's a slight chance we’ll buy your chapter as a short story. I might also say that in the twenty+ years I’ve been with On Spec, I can think of only a few times when we've bought what might have been the chapter of a novel. (Those stories were so well written I wasn't sure.) The one chapter we did publish was written by Dave Duncan, and we asked him if we could use it for promotion. Therefore, if you’re already a famous author, or experienced enough to know what a chapter needs in order to turn it into a shorter piece, don’t waste time sending us portions of your larger work.
Usually, the biggest problem with novel chapters masquerading as short stories is one of complexity. Because a writer may be loath to drop any character that will later develop his plot, the story ends up with too many characters. A big cast is expected in a novel; longer work affords the reader time to get to know each one. In short stories, too many characters clutter the stage. The reader wonders why such a big group is there, what they contribute. It may be hard to keep all the names straight, especially if they're alien or imaginary ones. Too many characters are a give-away that the story is really a chapter in hiding.
Back story can also present problems. Whereas the novel takes its time and readers are happy to be filled in on who is who and what is what, this level of detail bogs down shorter work. The short story is a more immediate form, taking place in a shorter period of time. Sometimes, the writer isn't aware he's writing a novel disguised as a short story - some ideas are too big to be contained in a smaller format. Whenever a plot covers years in passing, an over abundance of exposition may also be a mistake. The short story isn't really a short story. It's become a synopsis.
Chapters posing as short stories often have hanging ends. Although a good short story should have a sense that ‘the story continues’, there is always a sense of completion. The reader is satisfied; there are no questions remaining by the close. In a novel chapter disguised as a short story, the end is undetermined. There is a sense that the characters and plot are heading in a new direction. This is especially apparent in stories that introduce a quest, a birth, an exile, a solemn vow of vengeance, etc.
If you still feel your novel chapter might work as a short story, make sure you rewrite it to fit the shorter format. Narrow your cast and make sure the plot isn’t too convoluted. Build the dramatic tension in steps to a satisfying climax, and tie up all loose threads by the end. Camouflage it so cleverly, that even an editor won't know for sure.