Preheat the oven to 350 degrees F (175 degrees C).

Beat eggs in a mixing bowl. Add beef, onion, milk, and salt; mix well. Sprinkle in cracker crumbs; mix until combined. Form into 40 meatballs; place in a 9x13-inch baking dish and set aside.

Whisk ketchup, brown sugar, vinegar, and mustard together in a bowl; pour over meatballs.

Bake in the preheated oven until meatballs are no longer pink in the center, about 1 hour.