Beat the eggs in a mixing bowl. Add the onion, beef, milk, and salt; mix well. Sprinkle in the cracker crumbs and mix until evenly combined. Form the mixture into 40 meatballs and place into a 9x13 inch baking dish; set aside. Whisk the ketchup, mustard, vinegar, and sugar in a bowl; pour over the meatballs.
Bake in the preheated oven until the meatballs are no longer pink in the center, about 1 hour.