The MQL5 Market is the storefront built into MetaTrader for expert advisors, indicators and signal subscriptions, billed through an MQL5 account rather than through your broker. Products are bought outright or rented by the month, and every paid product ships a free demo build that runs only inside the strategy tester.
That restriction is the important part: the only evidence available before purchase is a backtest you run yourself, on your own broker's history and spread. Vendor screenshots and the star rating are not verification, published equity curves are frequently curve-fitted, and a purchase is licensed to a limited number of activations. Run the demo in the tester at realistic cost assumptions, then forward-test on a demo account before anything touches live money.
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Guide
How Execution Models Work: ECN, STP and Market Maker
Market maker, STP and ECN describe where the other side of a trade comes from — and what each model changes about spreads, requotes and order handling.