Is selling on e-commerce websites, which withhold funds for days and invest them in interest-bearing bank accounts, considered cooperation in sin and transgression, with the seller's knowledge, even though this matter is not mentioned in the contract?

The basic rule is the permissibility of dealing with them as long as the contract does not stipulate anything forbidden. Dealing with someone whose wealth is mixed, containing both lawful and unlawful elements, is disliked (makrooh) but not forbidden (haram). However, online selling of an item described but not present (Mawsoof fi al-Dhimmah) is not valid unless it is a Salam contract, which requires the full price to be paid at the time of the contract. This condition is not met in this scenario, as the intermediary website collects the price and does not transfer it to the seller until after a period of time, thus failing to fulfill the condition for the validity of a Salam contract.