 Shares of Carvana Co. CVNA, -12.58 jumped 2.4 in premarket trading Tuesday, after the online used-car retailer said it adopted a shareholder rights plan, in an effort to block investors from buying up a stake while the stock price is depressed. The stock has tumbled 61.5 over the past three months through Friday, and plummeted 95.5 over the past 12 months, amid liquidity concerns due to falling used-car demand and and reports of continued layoffs. 