Black market will use / sell them if they have the accessories to go with them
Yes correct the problem is not only the machines it is all the tubing valves and masks that are also required so the accessories that are needed to go with a machine to make it usable are a problem as well, and that will depend if it is invasive or not invasive as to what is required for each person being treated
A basic machine itself could be built around a high quality PLC it could be programed to do the most basic of breathing requirements, but could not be used for intensive care, this is the part that most people don't understand
So in the USA GM has started manufacturing parts for one company Ventec nothing they make will have to be passed, Ventec will be making all the parts that are already approved so any manufacture could be doing the same they just need to team up with a ventilator manufacture and make all the none critical parts mostly Injection molded and extruded parts are needed and stands Etc
If I had the molds required even for just one part I could have my molding machines running 24hrs a day as they can run unattended