How to manually set entity primary key in Entity Framework code first database?

Well, I have the following model structure: I have one class - DatabaseEntity which is basically

public class DatabaseEntity
{
    public int Id { get; set; }
}

so each entity like product, category etc will inherit DatabaseEntity and have Id property. Also I have typical EntityFramework repository class with InsertOrUpdate method:

private readonly DbContext _database;

public void InsertOrUpdate<TObject>(TObject entity) where TObject : DatabaseEntity
{
    if(entity.Id == default(int))
    {
         // New entity
         DbSet<TObject>().Add(entity);
    }
    else
    {
         // Existing entity
         _database.Entry(entity).State = EntityState.Modified;
    }
    _database.SaveChanges();
}

Then I download from eBay via eBay api list of categoies I have to add to database. Basically category is:

public class EbayCategory : DatabaseEntity
{
    // It has Id since it inherits DatabaseEntity
    public string Name { get; set; }      
    // ... some other properties
}

But, the problem is, when I download those categories I download and their Id properties, which, of course, already have values. And when I try to save them to database like:

public void UpdateCategories(IEnumerable<EbayCategory> newCategories)
{
    foreach (var newCategory in newCategories)
    {
        _repository.InsertOrUpdate(newCategory);
    }
}

I face some issues... First of all, entity.Id != default(int) because it has value, so repository tries to update this entity, instead of adding, but it is not in the database or context so it throws the following exception:

System.Data.Entity.Infrastructure.DbUpdateConcurencyException
"Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded. Refresh ObjectStateManager entries."

... because it thinks that someone else deleted entity which I am trying to update. How can I save this InsertOrUpdate logic, since a lot of projects are based on it, and be able to add items (EbayCategories) with primary key (Id) to database and then update/delete them like other entities without discarding EbayCategory.Id value?