Please don't use Borax. Borax is very toxic. Bleach degrades completely, but Borax remains toxic, and will even poison the soil and any plant life it comes in contact with. Sevin is no longer on the approved formulary for use with poultry, though some folks still use it.
Mites will laugh at DE, and invite their friends to join the orgy. They live for a long time in every little nook and cranny of the coop. Your best bet would be to walk away from this sale. If it's too late for that: What is it constructed out of? I think I'd try a pressure washer with a...