Example: Selling six pieces of the same items for two items each.

Batch trade (one vs two): Error, too much trade space.
Bag trade (six vs two each): Works fine

I'd really like to use batch trades but, really, it only has drawbacks.

I noticed. The whole item limit system will be revamped. The new system will let you temporarily sell over the limit but disallow you from new deposit and creating new trades if you are overloaded.