Heat a nonstick skillet over medium heat; cook and stir turkey, onion, bell pepper, and tomato until turkey is crumbly and no longer pink, about 5 minutes. Stir in ketchup, barbeque sauce, mustard, vinegar, celery seed, black pepper, and red pepper flakes. Reduce heat to low and simmer for 10 minutes, stirring occasionally. Serve turkey mixture on toasted hamburger buns.
