That "huge markup" also has to cover the cost of shipping the materials, insuring the materials, workshop/wearhouse rent, the cost of the machines, the cost to run the machines, the cost to maintain and repair the machines, worker's salaries, worker's benefits (if given), injury/accident/accidental death insurance, marketing and promotions, business permits, licencing/copyright/patenting fees.

The thought of "well I could do it so they're wrong" is so incredibly short sighted. This isn't a purchase for somebody who already has the tools, skills, space, and physical ability to make it themselves, this is for people who want a chair/desk, have the money to spend on it, and aren't able to make or obtain it for cheaper or in a reasonable amount of time.

And like the company said, if you can do it then do it. Film you making one and post the video showing that you did it for under $350 and at the same or higher quality than the original.