The major leagues, whatever else they may be, are not good proof of the efficient markets hypothesis. If they were, Seattle wouldn’t be the worst team in the sport, and Mets minor league slugger Val Pascucci would be starting against left-handers for some contending team. But however irrational individual teams and their decisions can be, the best way to tell what a team is going to do is to deduce what its interests are, and assume it’s capable of following them.

As baseball’s trading season nears, there is always much speculation over which teams will be buyers and which will be sellers, which entails psychologizing, and gets away from the fundamentals at work. To repeat a bit of what I wrote yesterday in addressing which American League teams should be looking to improve, that’s a shame, because those fundamentals are on open display.

The two really important pieces of information needed to judge whether a team should buy or sell are its winning percentage and the difference between how many runs it has scored and allowed. (This statistic, called run differential, is in many ways a better indicator of how good a team really is, rather than its record.) There are four types of teams: Those that have a good record and a good run differential, those with bad marks in both areas, and those that either have a good run differential masked by a bad record, or the reverse.

One reason I think we’ll see an unusually active trading season this year is that so many National League teams have both a bad record and a bad run differential — Washington, Pittsburgh, Houston, Cincinnati, San Francisco, San Diego, and Colorado all have bad records, and all of them have been significantly outscored, meaning their bad records likely aren’t flukes. Not all of these teams are likely to hold fire sales, and some of them have nothing worth trading, but all were at least 71/2 games out of a playoff spot going into last night’s action, and in positions where they’d be better off regrouping for next year.

To whom these teams (and the similarly situated squads in Seattle and Kansas City) might be peddling their wares is, of course, the interesting question.

The teams with perhaps the most incentives to add reinforcements are Milwaukee (42-34) and Florida (40-35). Both are within a couple of wins of a playoff spot, despite having scored as many runs as they’ve allowed. If they can actually improve their underlying performances, they should be able to sustain their records and remain in the hunt deep into the year. Both teams also have pretty obvious, oddly similar, and fairly modest needs: An average catcher, a decent fifth starter, and a reliever would measurably improve both teams.

After these two would be two more teams whose records aren’t in line with their performances: Atlanta (38-40, despite the third-best run differential in the league) and Los Angeles (35-40, despite an even differential). Both should improve even if they hold steady, but even marginal improvements could give them real boosts in eminently winnable races. The problem for Atlanta is that it will be hard for them to add any real players — their only true soft spot has been in right field, manned by 24-year-old Jeff Francoeur, who still might become a star and certainly isn’t going anywhere. Los Angeles has a somewhat similar problem in that they’ve been losing two of three for a month mainly because of injuries to star players such as Rafael Furcal, who should be back sooner or later — trading off young talent to fill holes they’ll fill as players come off the disabled list might not be a great idea.

The final teams that should look to improve are two teams with basically even run differentials and .500-ish records: Arizona and the Mets. As everyone knows, the Mets have basically nothing to trade, meaning they’ll either have to fleece someone or add to their already bloated payroll by picking up talent in a straight-out salary dump. Arizona, though, while it sent off a large chunk of its farm system for ace Dan Haren this winter, still has plenty of young talent, if few roster spots that might be realistically be improved. (Their two worst players to date, outfielder Eric Byrnes and 90-year-old pitcher Randy Johnson, are the two best-paid players on the team.) The other teams in the league — Chicago, St. Louis, and Philadelphia — all have their holes, as all teams do, but none needs to go all-out for fresh talent.

Put together the picture in both leagues, and this looks an awful lot like a market that should have more sellers than buyers, and few buyers in desperate need. As said, baseball isn’t necessarily rational, so that doesn’t mean we’ll see an epic month of madhouse trading, or even any at all. But if things work as they should, July could be exciting and fast.

tmarchman@nysun.com