Those considering buying eBay (ticker: EBAY) stock need to make this gut check: Are you willing to risk a 17% loss to make nearly 50%?

Earlier this year, activist investor Elliott Management announced a stake in eBay. The hedge-fund manager wants the company to sell or spin off ticket-reselling business StubHub and eBay’s classified-ads businesses, as well as pay a bigger dividend. On Friday, the two sides signed a cooperation agreement, and eBay will undertake a strategic review of its operations.

As...