Pongtastic!
(The website I built from scratch is still live, though hidden, here)
Lessons Learned
The time-value of money. With each ball taking between 2 and 20 minutes to make depending on the complexity, this was not a cheap process.
Buy low, sell high. The balls and markers were cheap enough to turn a profit, but if I valued my time at anything more than $0.30 an hour, the margins weren't too great here.
Have fun! Although this wasn't practical for becoming a millionaire, it did make me a lot of friends. I still receive appreciative photos of friends' (ping-pong!) balls.