Am I sinful for staying in an apartment that I bought from a tenant who had rented it from its original owner without specifying a period, then the tenant bought it from the owner for a meager price, and I paid the actual price to the tenant?

The duration of a lease must be specified to avoid ambiguity. If the duration is not specified, such as in a month-to-month lease, the contract is not binding on either party, and either party has the right to terminate it. As for an owner selling their apartment for less than its value due to coercion resulting from unjust rental laws, although the original contract is valid, this action is considered as if their property was usurped. Therefore, their action is not binding on them, and nothing that arises from it is binding on them.