Is it permissible for a person to sell services online, such as writing and translation, that he does not possess or cannot perform himself, provided there is a prior agreement with a third party to supply these services when the customer requests them, given that the amount is not transferred to the service provider until the customer approves the order?

The work in question is governed by an 'ijara contract (leasing/hiring), and it has no relation to selling what one does not own. The worker in this case is considered a shared 'ajir (joint contractor/employee). It is permissible for the shared 'ajir to accept the work and assign someone else to perform it at a lower price, so that he may take the difference, provided two conditions are met: adherence to the quality standards of the work, and that the hirer (musta'jir) has not stipulated that the shared 'ajir perform the work himself. If the hirer has stipulated that, then it is not permissible for the 'ajir to delegate the work.