Why not simply buy one of the low cost packages new? Take a look at Freemill,Meshcam,Dolphin or even the Path module of Freecad .I deliberately didn't mention Bobcad. I don't know of any licence agreement that permits the sale of installed software to a subsequent owner.There is sometimes an escape clause if the business that owned the software is closing down and the purchaser agrees to buy the cam "department" of the company.