I have an invention that I don't want to patent because of the cost. Will I be safe to manufacture the product myself and sell it online?
Even before you patent, documenting that your invention is strictly your own is an essential prerequisite. Speak with a patent attorney before you make any final decisions, but before, while, and after you do, keep documenting.
At the end of the day, even a patent isn't a fix-all--it's always up to you to defend your intellectual property rights in court.