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

you must have a lot of equipment if you need to have a storage unit for just the boxes. Don't the boxes have a lot of air in them, can't you collapse them?

keep the foam. With the flattened boxes, you’d still save half the space. The biggest reason to keep the boxes is to be able to safely ship them then or take the pieces with you when you move. But of course, I would not keep the boxes for cheaper items under $500

Any time I ship something and I don't have the box, I double-box it and spend 30 min on making it sure it's safe and I wish I had the original box

I would add: original packaging is generally safe but easy to use new packaging that is just as safe. It just takes time and material, and likely ends up a bigger volume due to double boxing which will cost more. I boxed porcelain figures that you could drop from the 2nd floor without a scratch. Lastly, most plastic used for packaging are very harmful, never decomposing staff

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.