If you exercise your ISOs post-IPO, you can trade your purchased shares on a public exchange among a global forum of buyers and sellers. If you exercise and purchase shares pre-IPO, you will own shares of a non-public company, which you may or may not be able to sell to another party until the company completes its IPO. Even if you can sell pre-IPO shares, there may be significant restrictions, and it can be hard to determine their fair market value.