Infamous short-seller Andrew Left scrambling for ways to bet against bitcoin

• Opportunities to short, or bet against, bitcoin are "limited," "but they're nice and juicy, and Riot's one of them right now," Citron Research founder Left says on CNBC's "Fast Money."
• Riot Blockchain turned down a CNBC request to appear on "Fast Money." "No one at the company has been able to get in touch with the CEO," a Riot spokesperson said.
• A major problem is being unable to borrow shares in order to sell them short, he said. If people could "get borrows" on the Bitcoin Investment Trust, Left says, it would fall 1,000 percent.

Short-seller Andrew Left can't find enough ways to short, or bet against, bitcoin.

"If you're a short-seller right now and you're cautious and you want to short something that's related to bitcoin and not bitcoin, you have limited opportunities," the Citron Research founder said Tuesday on CNBC's "Fast Money."

"But they're nice and juicy, and Riot's one of them right now," he said.

Citron Research tweeted Monday afternoon that it was shorting Riot Blockchain, a biotech company that added "blockchain" to its name in October and saw its shares surge. The stock is now up more than 900 percent this year and has a market cap of about $372 million.

Riot turned down a CNBC request to appear on "Fast Money." "No one at the company has been able to get in touch with the CEO," a Riot spokesperson said.

Based on his "sum of parts" analysis, Left said on CNBC the stock is worth $9 a share, down nearly 77 percent from where it closed on Tuesday.

Shares fell 6 percent in after-hours trading after rising 5.8 percent to $38.60 a share Tuesday.

Michael Newberg | CNBC

Andrew Left, Citron Research

Riot Blockchain announced Tuesday it raised $37 million in gross proceeds from subscription agreements with accredited investors for Riot shares, with Canaccord Genuity as financial advisor. The proceeds will be used for bitcoin mining and other investments, Riot said.

Left said he is using options to bet against Riot. He said the company is "even worse" than the initial coin offerings that have raised millions of dollars each based primarily on plans laid out in online whitepapers.

Riot has just enough blockchain-related investments to announce them publicly, "the whole time really misrepresenting the fact that nothing they have is a real player in the crypto industry," Left said.

The company used to focus on blood testing and animal health care. While Riot says on its website that it maintains its Bioptix business, the company intends to "gain exposure to the blockchain ecosystem through targeted investments in the sector, with a primary focus on the Bitcoin and Ethereum blockchains."

Challenges for betting against crypto stocks

A major challenge for shorting bitcoin-related stocks is not being able to borrow shares, Left said. Short-sellers typically borrow shares and sell them in the expectation they can buy back the shares at a cheaper price in the future.

Left previously announced he was short the Bitcoin Investment Trust (GBTC), which is traded over the counter, rather than in a formal venue such as the New York Stock Exchange.

"I think if other people were able to get borrows [on GBTC] it'd be down 1,000 percent tomorrow," Left said Tuesday.

However, some respected investors have pointed to the investment vehicle as a way to bet on bitcoin.

GBTC is up more than 2,300 percent this year and is the largest holding of the top-performing ARK Innovation Exchange-Traded Fund (ARKK) at nearly 10.5 percent as of Monday. Fundstrat's Tom Lee said in late October that the Bitcoin Investment Trust was an "attractive" buy.

Left also said he "couldn't get a borrow" to short Longfin, one of the latest stocks to soar on the hype around bitcoin and blockchain.

Longfin closed nearly 5.7 percent lower Tuesday after briefly soaring more than 2,500 percent between Friday and Monday after announcing it was buying blockchain company Ziddu.com. The company's CEO and Chairman Venkat Meenavalli said Monday on CNBC's "Fast Money" that "this market cap is not justified."

An increasing number of mostly tiny stocks have surged following news, or speculation, of the companies' involvement in cryptocurrencies or the blockchain technology behind them.

The U.S. Securities and Exchange Commission has become more active in cracking down on cryptocurrency-related speculation. The commission on Tuesday temporarily suspended trading in shares of The Crypto Company, partly on concerns of potential stock manipulation.

In a sign that the speculative mania is getting out of hand, one very small stock, Future FinTech, briefly leaped more than 200 percent Tuesday despite no clear ties to anything crypto-related.

Bitcoin itself briefly tumbled $1,000 in less than an hour Tuesday afternoon. But the digital currency has proved enormously resilient this year. It shook off several declines of more than 20 percent to rise 2,000 percent over the last 12 months to above $19,000, according to Coinbase data.