As evidence for that, Mr. Neiss pointed to the dozen or so companies that tried to raise money on the uFunding site, without the disclosures required by law. None of the companies raised more than a few hundred dollars before uFunding was shut down.

But the crowd did not manage to catch the problems with Ascenergy, a company in Nevada that raised $5 million on crowdfunding sites with a promise of creating an oil and gas business. The company used a less discussed provision of the Jobs Act that allowed it to raise more than $1 million from more sophisticated investors.

When the S.E.C. shut down Ascenergy in 2015, the agency said the company did not appear to have any of the expertise or contacts in the oil industry that it had claimed in its online material.

By the time Ascenergy was stopped, the company had already spent most of the millions of dollars it collected from 90 or so investors. Much of it was used for the personal expenses of its founder, paying for “fast-food restaurants, Apple stores and iTunes, dietary supplements and personal care products,” the S.E.C. said.

A lawyer for Ascenergy did not respond to a request for comment.

Mr. Feit, whose site had rejected Ascenergy, said that he was less worried about outright frauds and more concerned about companies that were unlikely to ever pay off and that were not giving investors enough information to judge them.

Mr. Feit has been particularly worried about companies that have assigned themselves sky-high valuations that will make it hard for investors to ever make their money back. In several cases, companies that he rejected because of their high valuations have shown up on other sites with the same valuations.

Mr. Feit and Mr. Tyrrell have been leading an effort through the Crowdfund Intermediary Regulatory Advocates to create a set of industry standards that all crowdfunding sites will adhere to when they vet potential investments, which will include a consideration of valuations and compliance.