Answer
The buyer should take into account not just one “purchase tax,” but the entire set of ownership-transfer costs: applicable taxes, registration fees, certificates, and the escribano’s fee. The calculation may be based on the car’s official value, not only on the price stated by the parties in the listing.
After the purchase, there is an annual patente de rodados, the amount of which depends on the car’s characteristics and assessed value. It should not be confused with one-time transaction costs. Before signing, the escribano should check the former owner’s outstanding debts and calculate the new owner’s payments.
Ask for an estimate before making a deposit and specify which party pays each item under the contract. Do not budget a fixed amount based on someone else’s transaction: a new car, a used car, and registration in different departments may produce different totals.