Registering used car

The special seller's CT is mandatory if the car is sold to an individual. Without the special CT (valid for 1 month), you won't have the paperwork to re-register the car.
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In practice, you agree the sale, the seller goes off and CTs it, and the buyer goes off and gets their number plate + insurance + payment. Meeting 2, you swap cash, paperwork and car.
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Always write up a contract of sale with the vehicule chassis number and both buyer and seller's ID card numbers.