He started out with 22,203,018 shares and before the latest purchase had 21,888,416 shares, for a total of 314,602 sold shares (or a mere 1.4%). If anything that's a surprisingly little amount of shares sold considering it is the bulk of his net worth. Note that he's a billionaire if you count the equity in Square, his Twitter holdings come out to $645M (counting a cool $54 net gain on his TWTR holdings today!).
Though he almost certainly got cash before too -- DST bought $800m of shares in 2011: half new shares, half from founders and employees
http://www.mercurynews.com/wiretap/ci_18596988?source=rss
plus he had 23.4mm shares pre-ipo
http://www.businessinsider.com/the-folks-cash-flushed-from-t...
And why would he have to sell shares during the ipo? As far as I understand, he should have had virtually no expenses, assuming he (as all founders should) did an 83b election early on.
Either way, he should have been taxed at ltcg, so at most 20%.