How to restrict vendors from posting on multiple categories for the same products on nopcommerce 4.0

You will need to modify the ProductController.cs in Nop.Web/Areas/Admin/Controllers/ where it populates the product category

Let say you have SALE and GIFT categories and you don't want your vendors to map their product to these categories then you can perform the following.

Assuming that your vendor that does not have access to "Admin area. Manage Categories" from Access Control List

Add a permissionService as shown in the code below.

protected virtual void PrepareCategoryMappingModel(ProductModel model, Product product, bool excludeProperties)
{
    if (model == null)
        throw new ArgumentNullException(nameof(model));

    if (!excludeProperties && product != null)
        model.SelectedCategoryIds = _categoryService.GetProductCategoriesByProductId(product.Id, true).Select(c => c.CategoryId).ToList();

    var allCategories = SelectListHelper.GetCategoryList(_categoryService, _cacheManager, true);

    //check if user has manage categories access. if admin then show sale category, if vendor then don't show sale.
    
  //Vendor will have access to all product categories except SALE and GIFT
  if (!this._permissionService.Authorize(StandardPermissionProvider.ManageCategories))
    {
        foreach (var c in allCategories.Where(x => x.Value != "2127" && x.Value != "2128")) //Where Category SALE = 2127, GIFT = 2128
        {
            c.Selected = model.SelectedCategoryIds.Contains(int.Parse(c.Value));
            model.AvailableCategories.Add(c);
        }
    }
    else
  //Admin will have access to all product categories
    {
        foreach (var c in allCategories)
        {
            c.Selected = model.SelectedCategoryIds.Contains(int.Parse(c.Value));
            model.AvailableCategories.Add(c);
        }
    }
}

I guess in your case, you will need to create a logic wherein if the vendor is selling mobile phones (Mobile Category) then don't load the the other categories.

Hope this will get you going.