Okay, I deserved that one. ;) In my defense, people on eBay tend to keyword spam, so that simply looking at the shirts so tagged wasn't telling me anything. Nor were the labels on the shirts. And that's what I tried first: *looking* at them to see if I could tell what made those shirts 'board shirts'.
LOLZ... the Board Shirt has been made continuously since at least the 1950s, and is still quite popular with fans of surf culture, thanks to the Beach Boys wearing them on some of their album covers (the original name of the band was The Pendletones).
This album cover photo (from Surfer Girl -1963) shows the most popular colorway... I have a repro of this one from a few years ago. Vintage versions are sought after.