Is it permissible to supplicate for the sale of a specific number of cars or to become the best salesperson in order to earn an additional wage, or does this constitute an transgression in supplication?

There is no harm in supplicating with any legitimate formula to seek provision (rizq). It is preferable to supplicate with what has been reported from the Prophet, peace and blessings be upon him, such as: "O Allah, I ask You for beneficial knowledge, good provision, and accepted deeds." And seeking forgiveness is one of the greatest means of obtaining provision.