Storage boxes. Can I discard high end shipping boxes, and still sell in the future?

I'd say sensitive equipment boxes and packing should be kept for shipping unless selling locally. I'd never discard the box and molded styrofoam inserts for my tube integrated amp.

I acquired most of my equipment second hand. 
 

I never gave thought to whether or not it came in it’s original box. Most of the time, it did not. 

The "original box" thing is overblown for most things. it is something people have made up over the years and it makes no difference in most cases. Paying storage fees means you need to think about rearranging your life instead of throwing money away on storage fees, that's ridiculous. If you feel like you may sell your gear in a short time, yeah, save the boxes. I do save some boxes until I'm certain the devices work properly, but toss them soon afterward. McIntosh will sell you proper packaging if ever needed. For other brands you can pretty much just pack them properly if needed. In my case I have an authorized repair facility in town for all my brands so will never be shipping anything out for service. There is nothing on this planet that can't be packed properly for shipping with materials other than the original. This is not 1890, and most everything we deal with is not and will never be collectible and have the box matter.

Also, it's like buying a car. You are not putting the thing through an X-ray before you to assess if it was properly taken care of, so you assess the condition based on a lot of little things. Preserving the box is one of those and as said before, it boosts the price. 

My experience with buying and selling is that items that have original packaging make the market broader and in general garner a higher price and sell quicker. Many buyers won't consider buying without it. It is not because material is not available to pack items correctly. It is that you have to trust someone across the country that you don't know to properly pack an item for shipping without the original packaging. There are far too many that can't or won't properly package an item for shipment. It is not cheap to package delicate electronic equipment properly for shipment.

I am lucky in that I have a large, dry storage building and keep all packaging no matter how long I intend to keep an item. When possible, it is best to have it on hand..