How to handle string features in a neural network
Does the string add any useful data? No. You should replace categories by id's for sure, and manufacturers as well. Why would having uncorrelated groups of strings stop you from giving each an id? 

So Google = 0, Apple = 1, etc. But that doesn't mean that a category can't have id 0 or 1. This is a different input, so these should start at 0 too: Books = 0, Electronics = 1. You still have to normalize these id's for your network ofcourse (value = id / maxId). 

This might be useful: https://github.com/cazala/synaptic/wiki/Normalization-101