Depending on the purchase at 100 sales, could it be 'on the house', then blog about it (Facebook, blog, etc) thanking all to have contributed to your success and sharing excitement about what the next 100? I wouldn't, however, share details about the sale itself.
Here's what I did at 500 (over the moon, of course!):
http://customaed.com/products-page/celebrating-500-sales/lol, I guess I offered a discount for the month, too. But, that is not as memorable to me (obviously!) as the memory of how excited my 500th customer was to receive her order FOC.
And, congratulations!