Panning with karts is lot like panning with boats, because of the lack of suspension you can get some nice background look BUT because of the vertical movement of the vehicle you can get shots that you may not be as pleased with as you might in other motorsports.
It is sometimes better to shoot a bit higher shutter speed than you might do normally for this kinds of vehicles to mitigate this vertical motion and the resultant softness. It can pay off to look to find the smoothest part of track to grab some panning shots.
I am shooting karts and super moto this weekend but they aren't running on a track rather they are racing on city streets made up of 30 plus year old concrete paneled roadway along with a portion of the course recently repaved with asphalt. My panning shots will be coming from that portion of the course.
Remember to shoot low, a point of view from no higher than the drivers head makes a shot much more appealing and try to expose to pull the eyes out of the helmet.
As to sales, last year I passed something like 150 cards out to the racers and family members. In most cases I was approached by them to know if I had shots of a particular kart or bike, I handed them a card and advised them I would have the shots on my site by the day following and if they wanted a print or download an image to review them and contact me. I sold about $1,200 in the week or so following.
The tough part was sorting down a hundred or so bikes and karts into seperate folders making it easier for the clients to find the shots they were interested in but it certainly pays off instead of them having to muddle through the 1500 or so images I end up shooting in the multiple classes and days.
More money can certainly be made by being able to download and print on site but that requires multiple co-ordinated individuals, informed racers that these are available, the equipment and usually getting and paying for vendor space. I actually just go to shot for fun and watch the racing the money part is really secondary to me. But to bring in an extra couple bucks is worth the extra work in the evenings, but with the gf's b-day this weekend I may be less apt to spend the time I have in the past.
My usually choice of equipment is the 40D with the 70-200 f/2.8 and the 30D with the 24-105 f/4 for panning shots because I am usually stopped down anyway.
Hope this helps, have fun!
Here is a couple of the shots I got last year, a few more can be found in the motorsports folder on my zenfolio, link below.
In the first shot you can see her eyes are sharp but the kart is soft because of the kart bouncing through a rough spot.
This image is the inverse of the one above, in panning a nice effect and the kart is sharp but the driver is bouncing because of the lack of suspension so he is soft.