In principal, every user can develop his own EAs which will trade based on the programmed trading strategy, even automatically depending on the set up. A great number of Expert Forex pro systems ea – discussed and tested in various forums- are offered at no cost in the internet.
Often the “Open Source” concept still prevails which means the source code is disclosed and can be used and modified by the traders at their discretion. On the other side, commercial Expert Advisors are closed systems, the source code cannot be accessed by the trader. For these cases the buyer needs to rely on the seller’s information regarding the programmed trading system as long as any information is disclosed at all. Many Expert Advisor providers have moved to a stronger protection of their source code. Basically you should have understood the Expert Advisor’s input parameters and trading strategy before its use, regardless whether you use a purchased version or an EA freely available in the internet. EA and to try it on a demo account for a longer period of time, even if the results cannot be transferred to a live account offhand.