Enough has been written about estate planning in recent years to fill several editions of the Encyclopedia Britannica. But one bit of advice unavailable in all the tomes on this subject is that the best time to sell a business is right now.

As the stock market has risen to an all-time high, the prices paid for private businesses have followed suit. It’s not at all uncommon for closely held companies to command eight to nine times earnings before interest and taxes (EBIT), and some businesses fetch even higher multiples.

How long will private companies continue to be worth as much as they are now? That’s like asking how long the stock market will continue to go up. If history is any guide, the market sooner or later will stumble or, at the least, stagnate. In either of these eventualities, the prices paid for private companies probably will decline, perhaps sharply.

The likelihood is that not in the next two or three decades will private companies fetch prices as high as they do now. Thus it behooves any business owner in his 50s or 60s to give some serious consideration to selling now.

As an investment banker, I realize that I’m suspect in offering this advice because my firm profits when it arranges for the sale of a company. But in Southern California alone there are far, far more potential sellers than we could ever handle. My purpose is to alert owners to the certainty that this opportunity won’t last forever.

What would be the price of not selling now? As previously noted, owners can often get eight to nine times EBIT for their companies. Thus, a company with EBIT of $3 million might fetch $25 million today. But if interest rates should start climbing and the Federal Reserve raised its rates by, say, 150 basis points (1.5 percent), the stock prices of large companies might well decline 20 to 30 percent.

Under those conditions, large companies probably wouldn’t pay more than four or five times EBIT for private companies. Thus a company that’s worth $25 million today might command no more than $15 million. How many owners of private companies can afford to sit tight and risk seeing their businesses decline in value by 40 percent?

Now consider a more positive outcome. The owner sells now, taking stock rather than cash for his company in a tax-deferred transaction. Then he hedges his position to protect against a possible decline in the value of the stock.

A few months later, as expected, Congress lowers the capital gains tax rate. For illustrative purposes, let’s say it’s cut to 14 percent from the present 28 percent.

Multiply 14 points times $25 million and we see that our hypothetical owner saves about $3.5 million in capital gains taxes. For a 60-year-old owner who can expect to live to 80, that’s as much as $15,000 a month for the rest of his life if he has invested in fixed income securities.

Still another argument in favor of selling now is the current trend toward consolidation and vertical integration in many industries. The largest companies in these fields are looking for acquisitions across the United States, and they’re paying hefty prices.

But they don’t have to acquire every company in their industry to become the dominant player. Thus, the owners of private businesses should be aware of any trends toward consolidation in their fields. When they see this occurring, it will be to their advantage to preemptively approach the company doing the consolidating and offer to sell their companies.

The first company in any given geographical area to do this is likely to fare very well; competitors, in contrast, are likely to wither away in the face of competition from the consolidating company.

It’s certainly understandable why owners of private businesses are reluctant to give up control. For most, their companies are their life; parting with them would be like selling their first-born children.

Counterbalancing this consideration is the above-cited likelihood that not selling now would mean forfeiting millions of dollars. How many owners would be willing to confess to their children that their legacies were much less than they could have been if Dad hadn’t insisted on hanging onto his company?

Another reason business owners cite for not selling is, “I don’t want to shop the company around.” They worry that by making it known they’d like to sell, their businesses will somehow be tarnished or viewed as unstable. Customers may take their business elsewhere, valuable employees may quit, vendors may be less liberal in extending credit, goes the argument.

In my opinion, owners overestimate the dangers of shopping their companies. If it’s done correctly, trying to sell a company can be viewed as a positive rather than a negative factor. It’s well-known in the business world that there are many companies and investors looking for strategic alliances with strong, well-established businesses.

Undoubtedly, some owners will cite still other reasons sound reasons for not selling now. But those reasons will almost certainly pale under the harsh reality that they’ll never see higher prices. Those who don’t act now face the probability that they’ll miss the opportunity of a lifetime.

Peter H. Griffith is managing director and head of investment banking and equity research at Wedbush Morgan Securities.