When I read your concern, I had the same thought as Scottyb. When I used to race we used a product called a line lock to keep the car from rolling out of staging. It just locks the brakes and holds until you release by pressing a button or flipping a remote switch. They are around $25-$50.
Here is an electric one:
Here's a manual one:
On your tax issue, you will not lose any tax credits if you sell your cart. You only need to buy it, take delivery and after that you can sell it without recourse.