Place sausage in a large, deep skillet; cook over medium-high heat until evenly brown, about 10 minutes. Remove sausage from the skillet and let drain on paper towels. Combine melted margarine with the sausage drippings until there is 1 cup of drippings and margarine combined.
Cook and stir onion and celery in the margarine-dripping mixture in the same skillet over medium heat until onion is tender but not browned, about 10 minutes . Stir in about 1/3 of the bread cubes. Pour onion mixture into a large bowl and stir in remaining bread cubes, sausage, poultry seasoning, and pepper. Watch your hands, it's hot. Mix well. This stuffing is ready for baking.