Intellectual property that is not effectively protected can be legally practiced by anyone. It is very difficult to sell something that is available elsewhere for free. Some inventions can be protected by keeping them a secret. Inventions can also be protected by obtaining at least one broad patent (and preferably patents on improvements to the original invention) in each jurisdiction where one needs to be able to use that nation's legal system to prevent others from practicing the invention.
Trade secrets are enforced by preventing disclosure of the invention, by court action, if necessary. If the invention is "disclosed" as a result of its first sale, a trade secret strategy will not work. Patents are enforced by identifying and suing the infringer(s). If it is difficult or impossible to find out if an unauthorized party is practicing an invention (e.g., performing an industrial process), then patent protection is of marginal value and convincing that party or others to spend money to buy or license an invention is difficult. Moreover, if the market for the invention is in jurisdictions with unsophisticated (or corrupt) legal systems, patents in those jurisdictions are not very valuable. According to the Wall Street Journal, in 1995 the median cost of filing a patent infringement suit in the U.S. was about $350,000 and the total cost of prosecuting the case was about $800,000 - so only inventors with very valuable inventions and a source of patient capital can afford to sue.
A word on inventor paranioa is in order here. Some inventors, typically "independent" inventors who work alone, express and exhibit excessive concern about their inventions' being stolen. While infringement of extremely valuable patents is not unusual, the U.S. and other competent jurisdictions have in place laws that can be used by inventors to punish such behavior. As an inventor, one must take care to balance the urge to keep the light of his/her invention under a bushel against the fact that people do not buy things that they know nothing about. In U.S. and European courts, inventors win about 80 percent of the patent infringement suits that they bring - so the system works. Use it if your invention is valuable enough to justify the high cost of enforcement.