Back in 2006, Jill Starishevsky started a business selling license plates for strollers. The idea was that when your nanny was out with the baby in the stroller, people could anonymously report on her behavior (whether good or bad) via the website, HowsMyNanny.com, listed on the plate. So kinda like those "How's my driving?" signs on the back of trucks.
Her site is no longer active. It lasted until 2009, and has now been replaced by a spam site. But the original site is preserved on the Web Archive
I see two problems with her business plan. First, her customer base was limited to people with nannies. And second, I don't think the purpose of the license plate would have been evident to your average member of the public.