Republican congressional candidate Sean Bielat, who created a substantial nationwide campaign donor base in his failed bid to unseat Barney Frank two years ago, pocketed as much as $10,000 after the election by selling access to his highly prized list of contributors.

The problem was that there was no record that Bielat had the right to sell access to the donor list. Such rosters are owned by campaigns, not the candidates themselves. Bielat now says he purchased the list from his campaign in exchange for forgiveness of debt.

But his campaign committee never disclosed any sale — or erased the debt — until a year later when it went back and amended its campaign finance reports.

The episode illustrates a pattern of serious financial disclosure troubles brought to light in a Federal Election Commission audit earlier this year. Tens of thousands of dollars in donations, debts, and expenditures from Bielat’s last campaign were not reported. When the documents were ultimately catalogued, mistakes were made and reports contradicted each other, the commission found.

In June, Bielat agreed to pay a $2,750 fine for the accounting problems, and was ordered to send a representative to undergo FEC training.

Starting fresh

The misstep followed yet another publicly embarrassing episode in the 2010 race when the Norfolk Republican paid himself a $10,000 salary drawn from his political funds during the last weeks of the campaign against Frank, failing to publicly report the payment until the election was over. Bielat later said that he waited to reveal it to avoid what he said would be a distraction from his efforts to defeat the incumbent congressman.

The troubles leave a black mark on the image Bielat seeks to project, that of a competent businessman and an advocate for fiscal accountability and transparency in his District 4 primary battle next Thursday.

He will face former state mental health commissioner Elizabeth Childs, a Brookline School Committee member, and David L. Steinhof, a Fall River dentist. The winner of the primary is expected to face Joseph P. Kennedy III in the general election.

In an interview this week, Bielat acknowledged that chaos permeated his campaign finances in the 2010 race.

The candidate, whose run against Frank drew huge national attention and massive donations from conservatives around the country, says the problems first started with his campaign’s inability during that race to handle the surge of money that flooded into its war chest in the weeks leading up to the election.

That influx, combined with what Bielat calls the incompetence of his committee’s campaign finance consultant, created the beginnings of the fiscal disarray that led to the election commission’s reprimand.

He said he was taking full responsibility for not getting on top of the accounting mess as report after report was filed last year.

“I screwed up,’’ he said in an interview this week. “I should have done better.’’

This campaign season, Bielat said, he has assembled an entirely new campaign committee to handle his finances.

But he defended the profits that he collected from selling his list of contributors, saying the $14,300 debt he forgave his campaign in exchange for obtaining the roster is evidence that he abided by federal campaign finance rules.

Those laws dictate that fund-raising lists are owned by the campaign committees. Candidates may use them, but only if they buy them at fair market value and publicly disclose the transaction.

Because he was running against such a high-profile Democrat — and one so despised by conservatives — Bielat’s list would be considered highly valuable even now to any conservative who wanted to seek public office and was looking to identify donors.

Despite Bielat’s insistence that he followed Federal Election Commission rules, his committee never recorded the debt forgiveness deal in public financial disclosures from late 2010 to 2012. Bielat said the agreement was informal and without documentation.

But the campaign continued to carry the debt owed to Bielat on its books. In a February 2011 interview with the Globe, Bielat confirmed that the debt to him still existed.

A year later, however, his committee, in effort to justify the transfer of the donor list to Bielat, said it had forgiven the debt in late 2010 in exchange for the list.

Bielat said he could not explain the discrepancy.

At the time, Bielat told associates he had no plans to make a second run for Frank’s seat. But when the veteran congressman later announced he would not seek another term, Bielat changed his mind.

Only after Bielat decided to run and was required to fill out a federal personal financial disclosure form reporting his income, which included the money earned from the list, did he go back and amend two years’ worth of campaign finance reports.