Valve could prevent this by doing it like the Steam Deck and requiring an x year old Steam account with at least y game purchases on it to be allowed to order one. Businesses aren’t going to grab secondhand consumer hardware to save a buck, and even if they are the majority of Machine buyers wouldn’t be looking to sell (and the margin necessary to get someone to effectively put the price of a Machine on layaway then ship it to some business and pay taxes twice will probably erase any gains the company would possibly see from using Steam Machines instead of Optiplexes)