Do ISBN numbers really make a difference when purchasing books for college courses?

So the time has come to purchase books for next semester, and to save money I've been trying to purchase books from fellow students on Craigslist, instead of the college bookstore. Nevertheless, I find that the campus bookstore lists different ISBN numbers than the ones I'm seeing people post on Craigslist, but they appear to be the exact same book, same edition, same author. Will it make a difference, or is it just the bookstore's scheme to try and trick people into thinking they're the only ones with the correct books?|||If it's really the same edition, then the difference in ISBN is probably between paperback and hardback, in which case, either one is OK. But if there are different editions, you need to make sure you get the right edition. But you apparently are aware of that already.