Echoing what's said above, choosingwhether to sell Fisher mods is only about respect and nothing more. There are no legal issues associated because Fisher has not patented his designs. In fact I think almost nobody on twistypuzzles has ever patented their designs, although that doesn't mean you should go making and selling the designs of others.
As for buying DIYs... it is ALSO an issue of respect. I don't really respect Seven Towns as much, because although they have a good selection of puzzles, they haven't been able to make good 2x2 or 5x5 designs (or stickers) despite having spent so many years selling them, and their storebought 3x3s are still only good on occasion. Their target doesn't seem to be the speedcubing community (not that I blame them, though). On the other hand, I really respect Meffert's, because Uwe has many unique designs, produces high-quality and beautiful puzzles, and has historically been a great source for 4x4s and 5x5s. So I'd be a LOT less likely to buy a knockoff Pyraminx Crystal or Dogic than to buy a 3x3 DIY.
