As I understand it, she got a bunch of shares in Staples, before it went public. Romney's testimony was about the value of the shares. I'm guessing, but I suspect it was to determine how to divide the marital assets.
She then sold some or even all of the shares shortly thereafter, at the pre-IPO price. That means she sold it to another insider, since they weren't marketable.
But, the pre-IPO price was much less than the IPO price when Staples went public. Don't hold me to these exact numbers, but they were in the vicinity of $2.00 pre-IPO, and $19 post-IPO. So, she left a lot of money on the table. I should point out that the difference is typical: non-marketable pre-IPO shares are deeply discounted, because you can't easily sell them.
What happened next is kind of murky, but I've read that she has been filing one suit after another to try to recover that loss, and has spent millions (to her attorneys), to no avail. So, saying she is "bitter" is probably a gross understatement.
If Romney believed the stock over valued at $2.25 then it is very possible he made a large profit by selling the shares short. This is not only legal, it’s good business.