Some people associate prop guards with training wheels..... I rather liken them to anti lock brakes on a motorcycle.....lets hope you never have to use them, but if you do, they are there for you!
As for selling, no..... The design download is free for the taking, so if you have a 3d printer, you'll have them in no time. If you don't, find someone that does and print them out and enjoy. Just remember where you got the design from!
Thanks to all that like the idea. It works for me and I had purchased the original prop guards but didn't like to screw and unscrew. The other design with the snap on attachment to the original prop guards is a good idea too! However, I think the guy is charging way too much for them.