I have invented/created several items that I market. The first thing that many people will tell you is to protect your ideas via copyright, patent, etc. However, these "protections" will not prevent someone from stealing your idea and changing it marginally and marketing it. Unfortunately it is LEGAL- and legal does not necessarily equal moral and ethical.
One of the things that I created was a "diaper" for miniature horses -- it is a special bag for catching poop. Diapers are required in many towns when you participate in parades. I developed my own diaper because I couldn't find one for sale and not long after I offered them to my friends on a miniature horse forum, other vendors began to offer very similar products. I was like hummmmm
what's up with this. Now most sites that sell driving equipment for miniature horses carry mini diapers.
So I guess one of the most important things I can tell you is that once you make your invention public, expect that others will duplicate it in part or in whole.