Do not "only buy graded". Please, just don't.
Small flaws shouldn't dissuade anyone from buying anything. Or large flaws, for that matter. Are you only looking to re-sell? If so, then you need to learn grading, and how to get deals etc. If you're looking for personal collection, then do small flaws like these really bother you?
If you're buying from an "amateur", then yes, it's almost certain that you will find small flaws that were not disclosed. Even more-so if you're buying "collections" and not single books or small lots. It's just the nature of the beast.
If you buy collections, and want to submit them for grading and make sure you get the best results, then pressing prior to grading is always your best bet on comics with press-able flaws like this.