Donald Trump used his campaign funds to buy thousands of copies of his own book at retail cost, simultaneously diverting donor money back into his pockets while artificially boosting his sales figures. It’s a tactic that may be illegal, campaign finance experts say.

On May 10, the Trump campaign paid Barnes & Noble $55,055, according to a filing with the Federal Election Commission. That amounts to more than 3,500 copies of the hardcover version of Crippled America : How to Make America Great Again, or just over 5,000 copies of the renamed paperback release, Great Again: How to Fix Our Crippled America.

A spokesperson for the Republican nominee told The Daily Beast the books were purchased “as part of gifting at the convention, which we have to do.” Sure enough, delegates in attendance at the Republican National Convention in Cleveland in July were given canvas tote bags, stamped with the Trump slogan, and filled with copies of Crippled America, as well as Kleenex and Make America Great Again! cups, hats, and T-shirts. Delegates were also given plastic fetus figurines.

The Republican National Committee did not respond when asked to confirm that its presidential candidate was required to spend tens of thousands in campaign funds on copies of his own book. And a spokesperson for Hillary Clinton’s campaign, asked if they’d ever done something similar, said, “We think we’ve probably purchased a copy or two just to have in the office, but the campaign has never purchased her book in bulk or anything close to that.”

Paul Ryan (not that one), of the nonpartisan nonprofit Campaign Legal Center, said that Trump would have to forgo accepting royalties for sales on the book in order for the transaction to be legal, under Federal Election Committee rules.

“It’s fine for a candidate’s book to be purchased by his committee, but it’s impermissible to receive royalties from the publisher,” Ryan said. “That amounts to an illegal conversion of campaign funds to personal use. There’s a well established precedent from the FEC that funds from the campaign account can’t end up in your own pocket.”

When asked if Trump agreed to forgo royalties for sales of the book, the Trump campaign refused to comment on the record while representatives at Simon & Schuster, Trump’s publisher, did not respond to repeated requests for comment.

Federal campaign law dictates that campaign spending must not “result in the conversion of campaign funds to the personal use of the candidate or any other person.”

In 2014, the FEC issued an advisory opinion allowing GOP Rep. Paul Ryan to buy copies of his book The Way Forward from his publisher at a “discounted bulk rate”—provided there were no royalties paid to the Congressman.

“It may be the case for a candidate to instead donate those royalties to charity—that might be a permissible arrangement,” said the CLC’s Ryan. “But the bottom line is, no money of this $55,000 from the book can end up in Donald Trump’s pocket without violating federal law.”

For as long as Trump has been a presidential candidate, he has been finding ways to patronize his businesses and write it off as a political expense.

Previous FEC filings have revealed the Trump campaign spends millions utilizing Tag Air Inc., of which Trump is CEO, to travel by private jet. He also uses Trump buildings to house members of his staff and to host official events, and frequents the Trump Grill in the lobby of Trump Tower.

No other presidential candidate in history has had access to such a vast universe of self-titled business to frequent during their campaign.

Executives, celebrities, and politicians have long purchased copies of their books in bulk; many publishers even get high-profile authors to agree to buy a certain number of copies before a contract for the book is even signed. But Trump’s arrangement appears to be very different. Ben Bruton, who has worked in publishing public relations for 25 years, called the way in which the books were bought “suspicious.”

“What any author that I know would most likely do is go to the publisher and say, ‘I want a bunch of these in the goody bag.’ [The author would] come to the publisher and say he needed books for a charity or an event, and we would donate 500 all the time. And we’ll sell more to you at a 40 percent discount,“ said Bruton.

“The red flag, I think, would be that they’re trying to get back on The [New York] Times Best Sellers list, or they’re trying to buy them at retail for royalties.”

Bruton said Trump’s sales would only count toward best-seller lists if he purchased them from a brick-and-mortar store like Barnes & Noble.

“I’ve worked at four different of the major six publishing houses and a lot of times people want to buy a bunch of books to get on the list. You can’t do it from Amazon. You can’t buy them from the publisher,” said Bruton. “Only buying from place likes Barnes & Noble at full price is looked at as sales.”

The bulk purchase practice is not uncommon with right-wing campaign books. In 2014, a Tea Party PAC bought $427,000 worth of Senate hopeful Mark Levin’s then-four-year-old book Liberty or Tyranny in an attempt to make it a best-seller.

The curators of the Times’ list are hip to bulk purchases and weigh heavily against them, Bruton said. “That’s how the Times keeps that sort of in check, so you can’t just buy your way onto the Best Sellers list,“ he said. It may be why Trump’s book—which was released in November of last year—failed to chart on the Non-Fiction chart that week, or the Politics list that month.

“However, I do believe that was definitely an attempt to both make money and to get onto the best-seller list,” said Bruton.

Still, Trump has continued on the offensive this week, calling for a special investigation against his opponent, Hillary Clinton, and her family’s foundation, which he claims is “crooked.”

On a Monday morning edition of Fox & Friends, Trump called Bill and Hillary Clinton “grifters” and said that “the whole thing with them is a scam.”