Can you sell stocks worth millions of dollars?

On Friday, 2.59 million shares of Delta Airlines traded at an average price of $31.44 which means that $81.3 million dollars of DAL traded that day. So yes, you "can you sell stocks worth millions."

Note that large blocks of shares are often sold off over a period of time. They can also be transacted off exchange and then reported to the exchange.

Also note that when Buffett sold his shares earlier this year, daily volume was more than double what it is now.

Is it possible to sell stocks worth millions in an open market?

Yes. For high-volume stocks where millions or billions of dollars worth of shares get traded every day, you can easily sell shares worth millions. The liquidity is there. For example, $40 billion worth of AAPL shares traded last Friday (2020-09-04). Your million-dollar sale of stock will merely be a drop in the ocean.

For stocks that do not have such large daily volumes to support your million-dollar trades, you could still sell on stock exchanges by splitting your orders over a period of a few days (e.g. sell $500,000 on day 1, sell $500,000 on day 2, etc.).

Several news reports appeared about this but none of them mentioned who purchased those stocks sold by Warren Buffett, so I assume that he must have sold them in an open market.

Not all trades have to happen on stock exchanges. Institutional investors could do block trades, where they privately negotiate and sell a large block of shares to a buyer. They could also use crossing networks to maintain anonymity and execute their large trades without severe impact on the market price. Any combination of these methods could be used to sell shares over any period of time (e.g. sell $1 million in the open market, sell $5 million in a block trade, sell $2 million in a crossing network, etc.).