Branch Rickey, widely credited with being the most inventive general manager in baseball history, had a simple rule of thumb for making deals. He'd arch his bushy eyebrows, puff on his cigar and proclaim, "Better to trade a player a year too early than a year too late."

Sell high? That's just common sense.

The trouble, of course, is that ballplayers aren't stamped with an expiration date like a bottle of milk. There's no way of knowing exactly what distinguishes a guy who's just in a slump from one starting the downside of his career. Not to mention that in Rickey's day there was no free agency, no Wild Card and no playoffs between the end of the regular season and the World Series.

The modern general manager has to take into account so many issues before pulling the trigger on a trade. Selling high remains the goal. But it's not always possible.

"There are just so many factors," said a former GM who asked not to be identified commenting on players with other teams. "Is the player close to free agency? Are you in win-now mode or are you developing? If you're not a contender, when do you see yourself getting there? Do you have some surplus at that position? Obviously, if you sell high, you're going to get more in return. But every player goes up and down in performance. It's really a jigsaw puzzle."

Example: The Twins are in last place in the American League Central. Josh Willingham has 22 homers. He's been a hot name and would be an attractive addition for a contending team looking for a right-handed-hitting outfielder. Reportedly there are several teams interested, so Minnesota could expect to get a nice return if it moved him.

But ... Willingham is also signed through 2014. Even though he might bring a windfall in prospects, if GM Terry Ryan thinks his team can turn things around in the near future, that might not be enough to entice him to make the move.

Contending teams can face a similar dilemma. Take Giants outfielder Melky Cabrera, the Most Valuable Player of last week's All-Star Game in Kansas City. He spent the first four full years of his career with the Yankees, then was traded to Atlanta. The Braves released him after the 2010 season. He signed a one-year, $1.25 million contract with the Royals, had a solid season, then was traded again, this time to the Giants, for whom he's enjoying a breakout year at age 27.

The Giants are in first place in the National League West, trying to fend off the Dodgers. Cabrera has obviously been a big part of their success. It's highly unlikely they'd seriously consider trading him. At the same time, in the interest of due diligence, they have to at least put every option on the table. And if the decision were to be that his value will never be higher, that would be one way to fortify other areas for the stretch run.

That would have hidden pitfalls, too. Joe Klein has been general manager of the Rangers, Indians and Tigers. "It's not so much the trade, it's what you tell the rest of the team," Klein, currently executive director of the independent Atlantic League, explained. "Even though the trade may affect only one or two guys, you definitely send your team a message that can come back and haunt you.

"If you trade a Major League asset, it can send the message that you don't think the team is good enough to win. Of course, if you don't do something to strengthen the team, you can also send the message that you didn't think they're good enough to win."

Another name that has been thrown around in the days leading up to the July 31 non-waiver Trade Deadline is D-backs right fielder Justin Upton. Reportedly, Arizona could have sent him to the Red Sox for a package including Jacoby Ellsbury and Daniel Bard in the winter of 2010-11. The D-backs turned it down and Upton went on to have a huge season. So far he hasn't been able to match those numbers this year. Managing general partner Ken Kendrick called him "an enigma" last month.

Despite that, the D-backs seem to have taken the position that it will take even more to get Upton now than it would have a year and a half ago.

"We're in win mode," GM Kevin Towers told reporters.

But a big factor in whether or not Upton will be traded hinges on the front office ultimately deciding whether 2011 or 2012 more closely resembles what they can expect from him in the future.

Besides, just because a player is having a great season doesn't necessarily mean other teams will be willing to give up a boatload of prospects.

"Beauty is in the eye of the beholder," one high-ranking executive said. "Not every team sees a player the same way. Sometimes you're surprised to find out that you might like one of your own players a lot, but other teams don't regard him as highly.

"In a perfect world, in a vacuum, sure, you'd like to sell high. But more often than not, that's not really possible."