The electoral authorities should question the owners of the chain, said Irma Erendira Sandoval, a political analyst at the National Autonomous University of Mexico.
Wednesday's count will help determine whether there was fraud, she said.
"This is a fundamental stage where the transparency and cleanliness of the entire election is at stake," she told CNN en Español.
According to the group Alianza Civica (Civic Alliance), which seeks to ensure elections are free and fair, a poll of Mexican voters found that nearly 30% reported being exposed to at least one example of vote-buying or coercion.
Taking advantage of the fact that election returns are available online, critics began circulating copies of what they described as suspicious returns.
One such document shared by PRI opponents is an electoral return from a district in Michoacan. It appears to show that 362 of 732 registered voters cast ballots, but that the PRI won 945 votes.
For his part, Peña Nieto, in an interview with "NewsHour" on PBS, praised Mexico's democratic system and said he was putting his trust in the electoral authorities.
"We have electoral tribunals that will be responsible for addressing these issues and attending to these complaints, the ones filed by candidate Lopez Obrador," he said.
Ballots will be recounted in cases where the difference between the top two finishers is 1 percentage point or less; where the number of annulled votes is greater than the difference between the top two; where all the votes go to one party; and where there are obvious irregularities such as the votes not adding up.
Lopez Obrador must wait until Sunday to formally submit any application for a recount to the Federal Election Tribunal.
Avila, the youth activist, acknowledged that Peña Nieto will likely be confirmed as president, but contended that that was because of PRI's alleged maneuverings.
"The amount of evidence we present does not matter. It is a fact that Enrique Peña Nieto will remain," he said. "We have had a position since before the election that this was being imposed by fraud, and it's what we've been seeing."