2nd hand car

You will find a full explanation here http://www.xpats.com/buying-car-belgium
In brief, the seller makes a special technical control (valid for one month) and hands you this document together with the certificate de confomité, the Car Pass document and the old registration document, though I'm not sure the latter is necessary and a copy would do. You will receive the car without plates, so you have to make arrangements where to leave it until you get yours. Twice I left it with the seller for a couple of days,. Also write a sales contract (you can find them on the net), and check the person's ID against all documents. You can register and get your plates in person or register online, but the easiest is to get insurance as soon as you buy and ask the company to do that for you.

To add to what Mimi has written, because of the short life of the special control technique it is quite common to come to an agreement with a seller 'subject to passing the CT' and then the seller gets the CT done. The alternative is that the seller has the CT done in advance, doesn't sell the car within the one month validity and has to pay for another CT - potentially ad infinitum.

just stick with Cambio, it's so easy.