Answer
Under the trading regime described, the tax base may be calculated as income less allowable expenses; older guidance refers to a 3% rate on that difference. But it is important to understand which expenses may be included at all.
For goods trading, margin is not the same as the money in the account. You need documented purchases, cost of goods, import payments, delivery, storage, returns and proper records. Not every company expense reduces the tax base: salaries, company overhead and similar costs may not be included under this regime.
Before applying the regime, distinguish the roles in each transaction: purchase for resale, intermediation, commission or your own service. If the whole transaction amount passes through the account but income is only a commission, the contract should clearly distinguish the client’s funds from the fee.