One problem area I am finding is with FBA fees, you are a little blind when dealing with large volumes of products.  What would be real nice would be to get those Seller fees into Magento so you can perform real cost validations.

  £2.25, which includes a £1.21 FBA Fee.
  Amazon state that there is no algorithm to share so you can work it out yourself.

At least with this kind of information I could (cost+fba) and compare to my low price or whatever other strategy gets used.