Hey,
ABec is a industrial norm that tell you nothing more then the space between the balls and the inner and outerrings.
The higher the number, the more precize the bearing is.....
BUT IT TELLS NO S.HI.T ABOUT THE QUALITY..!!! its only a tollerance norm.
So IF you make for example a bearing out of wood with the same gradations as Abec 7 then you can call it abec 7 but I'm affraid that a wooden bearing will not work that well............
Then if you have a abec 7 or 9 bearing and you jump some stairs a few times, then the bearings will be Abec 3 or less, because the impact of jumping will make that the precission will be less and less
Better get some bearings made out of HIGH GRADE metals, that will make that the bearing stays in shape longer and will keep running smoothly