Car pass and certificate of roadworthiness

I don't know the amount exactly but it is not expensive. You need the car pass and a special "controle technique": make sure you tell them it is for selling the car. However, the documents are valid for only one month, so if you don't manage to sell it during that time you have to do it again, and pay again. Find a buyer first and then do the inspection. To avoid the buyer disappearing into the blue, write a proper contract specifying 100 euros down payment and your guarantee that you will refund it if you do not get the inspection done on a certain day, and that the buyer has to pay the balance on day x or the contract is null and void and the down payment gone.

http://www.car-pass.be/en/
http://www.car-pass.be/en/particulieren/index.htm
http://www.car-pass.be/en/particulieren/a-glance-at-the-car-pass.html
2 months validity
http://www.car-pass.be/en/particulieren/what-is-car-pass.html
http://www.car-pass.be/en/particulieren/frequently-asked-questions.html

That covers about everything

Agree the sale first, then whilst the money is being sorted out, you go and do the selling Controle Technique. They will give you the car pass and certificate you need.

If you sell to a dealer - you don't need to do a carpass - But you will get a lot less than a private sale as the dealer wants to make a hefty profit