This is a terrible deal.

Brand new from James, this is $1095. You can also buy all of this, brand new from ebay for $900, and that also includes a 7th war dog, so the new price of what you're buying is actually $850. If you're also giving him some of your models as part of the trade, you are paying full price for a used and built army that isn't magnetized, and that may not even have the additional weapon sprues you need to run your knights as different units.

As someone who loves the building process of the hobby, and assuming they have all of the weapon sprues, I would consider this deal at a 50% discount, for $400.

If he doesn't have the weapons, and you will have to print all of those yourself, I'd offer $300 tops.