The paint now scratches off very easily. Is there a sealer I can put on it to avoid this instead of repainting?
You can try any water based clear-coat; many are available at your local paint stores. Most likely this won‘t do any good as the paint itself isn’t stuck to the surface. Paint-Primer products really don’t have any “primer” in them, this is just a marketing gimmick. If you read the can’s instructions it will call for the use of a real primer. At this point there really isn’t anything you can do but strip it off or live with it, do touch-ups when needed.
I agree with the author about most paint/primers ability to prime, especially in your case with hardwood cabinets… however some applications will work, such as exterior unprimed wood… now in your case I would lightly sand your surfaces and then apply a coat of Pigmented Shellac Sealer Primer (avail at any paint store), this should SEAL IN the paint that is there… then finish coat with regular high grade interior semi gloss paint… Good Luck!