The idea is that thieves will be less likely to resell stolen merchandise if authorities can track them down.

The measure is part of a legislative package of 14 bills to combat retail theft in the state. The California Retailers Association says the issue has reached crisis levels, though it’s challenging to quantify because many stores don’t share their data.

The proposal would be voided if voters pass a tough-on-crime ballot initiative, which qualified for the November ballot on Tuesday but still needs certification this month by the Secretary of State.

Proponents, including district attorneys and some big box retailers, said the data collection proposal would shut down organized theft rings seeking to resell stolen goods and would close a loophole in existing laws that don’t require platforms to track offline transactions.

The rules under the bill would apply to sellers who make at least $5,000 profit and engage in at least 200 transactions in a year.

Opponents say the measure’s new requirement is so broad and vague that some platforms would have to start collecting sensitive information from all users, harming California’s e-commerce businesses.

“This is basically going to force businesses out of California,” said David Edmonson of TechNet, a technology advocacy group. “I imagine most sellers will have to think long and hard about whether or not they want to provide that information to the online marketplace just to be able to sell, you know, household products.”

Nathan Garnett, general counsel of OfferUp, a mobile marketplace that connects local buyers and sellers so they can complete transactions in-person, said the proposal would significantly benefit big box retailers and cripple classified ad sites’ ability to do business in the state.

In the case of OfferUp, its 11 million users in California would have to hand over their personal information before they could list something like a used coffee table or an old truck on the platform, Garnett said.

Opponents say the measure also runs contrary to a federal law that went into effect last July, which requires online marketplaces like Amazon to verify high-volume sellers on their platforms as part of an effort to tamp down the amount of goods being stolen from brick-and-mortar stores and resold online.

The federal law was negotiated to protect classified websites, and there was no legal loophole, said Carl Szabo, the general counsel of an Internet trade group NetChoice. The group, which represents companies including Facebook parent Meta and Etsy, filed a lawsuit against Georgia last week to halt the implementation of a state law that would establish similar requirements.

Requiring platforms to monitor all transactions, including those happening offline, is an impossible task, Szabo said.

Democratic California state Sen. Nancy Skinner, who authored the measure, said law enforcement needs the tool to go after professional reseller schemes, citing a case in San Diego in which a couple made roughly $8 million from selling stolen goods online. Online marketplaces are also already collecting information from users through the privacy policy they have to agree to in order to use the platforms in the first place, she added.

“The only people they would have to get that information from are high-volume sellers, not every single person who uses their site,” she said.

The proposal is part of a legislative package that would increase penalties for organized crime rings, expand drug court programs and close a legal loophole to make it easier to prosecute auto thefts, among other things.

Lawmakers are racing to deliver the bills to Democratic Gov. Gavin Newsom in a few weeks. Once signed, the bills would take effect immediately — a new get-tough-on-crime strategy in an election year seeking to ease the growing fears of voters while preserving progressive policies designed to keep people out of prison.