Some people earning more than $200,000 a year also face a 3.8 percent Medicare surcharge on investment income.
As a further incentive to sell, companies and private-equity buyers have piles of cash and financing for deals is widely available, investment bankers say, driving valuations for some sectors to their highest since 2008.
But owners starting the sales process now may find there is not enough time. Others may choose to keep businesses in the family for personal reasons that outweigh the possibility of a bigger tax bill later.
"The vast majority of companies are deciding they're not going to sell," said Cascadia Capital LLC managing director Christian Schiller, who advises family-owned companies for the Seattle investment bank. "Family companies are driven by a number of legacy issues, and taxes are one of the smallest."
Even so, some wealth advisers, who benefit from sales that generate windfalls of cash to invest, are warning clients about the perils of waiting.
Northern Trust Co, one of largest providers of estate and investment advice to multi-millionaires, says a family selling a $70 million business on December 31 would pay $10.5 million in gains taxes. On January 1, that tax bill could jump to as much as $20.2 million.
"That's real money," said Mary Ann Sisco, head of client solutions at Northern Trust's wealth management division.
The biggest obstacle to getting a business sold by the end of the year: time. Examining the books of a business for sale and drumming up cash can take prospective buyers six months.
"Unless you're already on the sales block, you're probably out of luck," said Holly Isdale, a former Lehman Brothers and Bessemer Trust adviser who formed Wealthaven LLC in 2010 to counsel ultra-rich families on estate and tax planning.
Many non-financial issues can also scuttle deals. Parents and grandparents, for example, may become emotionally invested in a company and see their business as a legacy they want to pass on to heirs.
"People say 'I've given my life to this company. How can I put a dollar sign on it?'," said Dennis Jaffe, a professor at Saybrook University in San Francisco.
Jaffe, who also runs a consulting business that advises wealthy families, said many families struggle with cutting ties. In some cases, families inflate their asking price, deterring serious buyers.
There are external factors, too. Choppy markets, continuing weakness in the U.S. economy and other storm clouds may derail deals. Robert W. Baird investment banker Howard Lanser said worries about Europe's debt crisis last month briefly put some deals on hold.
What's more, valuations for smaller companies -- those under $100 million -- or those in more cyclical businesses, have not yet fully recovered. Owners in these cases are more inclined to hold on, bankers said.
Some owners will conclude they will be better off keeping their business as a vehicle for building new wealth as well as generating income.
"There's a back-to-basics movement in wealth management," said Mindy Rosenthal, a managing director of Campden Wealth, a global ultra-rich family networking group. "You're not going to create or re-create your wealth by playing the markets."