MartijnGizmo,
That M3 shot is a stunning picture and I share SDK's opinion as well, if the owner sees your pic he will probably buy it as well. It's really worth the effort try taking pannings at 1/25 and it might be duable as well. A lot of practise will pay off and if you look at your pictures it cristal clear that you know what you're doing 8) Nice one.
I have sold pics before by showing the owner the photos on the lcd screen, I once showed a porsche driver a few of my pics, the first thing he asked me was how much he had to pay for them

Easy money
Keep up the good work guys
Cheers