In a March filing, MercadoLibre provided a "sensitivity analysis" showing how much a devaluation would potentially erode profits. As reported last year under an exchange rate of 6.3 bolivars per dollar, Venezuela generated so-called segment profit of $55 million. But if the exchange rate had been, say, 18.72 bolivars per dollar, the segment profit would have been just $14.7 million. That indicates the entire company's operating income would have fallen to $113 million in 2013 from $130 million in 2012.
But again, even the anecdotal rate of 18.72 bolivars per dollar may not reflect the currency's true weakness, given the company hasn't made any foreign exchange transactions at that rate.
Of course, MercadoLibre has a strategy for preserving the value of cash held in Venezuelan currency. Last year, the company bought two office properties in Venezuela for about 450 million bolivars, more than the profits in made in the country.
But piling into real estate in a country with rapid inflation is hardly a quick fix. If inflation continues, rent income may be eroded before the company can pass on higher rates to tenants. And if the government responds with interest rate hikes to address inflation, it could hurt the economy and demand for commercial property.
Argentina, where MercadoLibre generated another 25 percent of its segment profit in 2013, is another region investors should watch closely. The company itself points out in a filing that during January 2014 alone, the Argentine currency weakened 23 percent from 6.52 pesos per dollar to 8 pesos per dollar. If that inflation continues or MercadoLibre becomes unable to convert pesos to dollars, a situation similar to Venezuela's could arise.
(Read more: Venezuela inflation tops 57 percent amid protests)
There is an argument to be made that a weaker peso could help MercadoLibre because 40 percent of its costs are denominated in the currency. But Michel Morin of Morgan Stanley points out that inflation in Argentina may offset much of that benefit. The company employs many skilled workers and executives within the country whom it may need to pay higher salaries as prices rise. And as in Venezuela, tighter fiscal or monetary policy could quickly drag on the economy.
All of this makes it tough to get a good read on MercadoLibre's underlying performance. A better way may be to look at growth in items sold, which decelerated from 25 percent in the third quarter to 20 percent in the fourth quarter. With MercadoLibre trading at an eye-watering eight times 2014 estimated sales, well above the likes of eBay and or Amazon, investors may want to wait for a better deal on the shares.
—By CNBC's John Jannarone. Follow John on Twitter