Michel,
I can only tell you of my experience importing a Pantera from the USA to Canada. Hopefully this will be somewhat similar for you ...
1) when you agree on the terms of the sale with the seller, you should have them scan and email you high res copies of the TITLE and the Bill of Sale. The latter you can Google or just make one up. You don't need to pay for a Bill of Sale. I used this one:
Bill of Sale2) a MINIMUM of 3 business days before the car will reach the border crossing you plan to export through, you MUST have the title document faxed to *THAT* US Customs office so they can satisfy themselves the car is not stolen or being illegally exported (against liens, owed taxes, etc.). You MUST keep proof that you did fax them, as they are hard to reach by phone and may not tell you (even if you ask) that all is OK (although I called and was told all was OK). If you choose to export via a brokerage firm, this will be handled for you.
3) At the time of export, US Customs must review, approve and STAMP the Title as OK for Export. Without this stamp, Canada will NOT allow the car entry. If possible, have access to the seller in case there are questions. MAKE SURE that the title documents have been signed by the seller (usually on the back there is a transfer section) as without that signature, you will not be allowed to export. Again, if you use a broker, this SHOULD be covered for you.
4) Make sure you keep tidy records as you will be asked to produce the Bill of Sale and PROOF OF PAYMENT AMOUNT. I was held up at the Canada-USA border while they confirmed that I paid what I claimed I paid. This could get tricky if you get an exceptionally good deal on the car you buy. Not a bad idea to have your bank statements in hand and your banker on the ready.
5) After the car is cleared for export, you be asked to pay whatever taxes that apply in Belgium. Do your homework on this one. Here in Canada, I was asked to pay the Goods and Services (GST) Tax - which I was expecting - but they also hit me with a Duty Tax of 6.1% due to the car being manufacturered in Italy. Wrong! I paid the amount, but with the help of Mark Charlton on this list (who imported his Mangusta after I brought 3528 home), I applied for a refund of the Duty I (incorrectly) paid and was sent a nice-sized cheque in the mail! Bottom line is to be prepared - and assume that you may have to educate your border patrol!
This all may sound like a daunting task, but it's really just a step-by-step process. All you REALLY need from the seller is the Bill of Sale and the Title - and, of course, the car when you can arrange for shipment. I suggest wiring the money to your seller as there is a good paper trail that can back you up at the border. Of course, you will have to gain a rapport and some trust with your seller in order to ensure you don't get ripped off. That's what this forum is all about ... make sure you use local resources to inspect the car and give you objective feedback. Take your time. It will all work out in the end.
Feel free to PM me if you have any specific questions.