Hawzah News Agency – It is befitting for a trader to learn the laws (aḥkām) of buying and selling concerning issues that he commonly encounters. In fact, if he would be at risk of committing an unlawful (ḥarām) act or abandoning an obligatory (wājib) act as a result of not learning the laws, then it would be necessary [not just befitting] for him to learn them. It is reported that His Eminence Imam al-Ṣādiq (ʿA) said: ‘One who wishes to engage in buying and selling must learn its laws; if he were to buy and sell before learning its laws, he would fall into ruin by means of invalid (bāṭil) and dubious transactions (muʿāmalāt).’

If a person does not know whether a transaction (muʿāmalah) he has conducted is valid (ṣaḥīḥ) or invalid due to him not knowing the ruling (masʾalah), he cannot have disposal over what he received in the transaction nor what he handed over; rather, he must learn the ruling or exercise precaution (iḥtiyāṭ), albeit by means of a settlement (muṣālaḥah). However, if he knows that the other party consents to him having disposal over the item even though the transaction is invalid, then having disposal over it is permitted (jāʾiz).

Source: https://www.sistani.org/english/book/48/