Unlike the NBA, the NFL and its teams frown on their best players skipping from town to town while in the prime of their careers. So it's worth understanding that the best players slated for this year's free agency already have been taken off the market through franchise tags and multi-year deals in advance of Tuesday's kickoff.

What's left are a few gems and a whole lot of question marks, red flags and older name players well past their prime. Judging any deal obviously depends on the compensation involved. But as a rule of thumb, here is one person's potential pitfalls to avoid and seven players to keep an eye on:

The rules in 2013

Over 28 in 2013? Move along, pal

The typical NFL player declines too rapidly and sometimes without warning. Signing older players can saddle a salary cap with too much "dead money" that's still on the books while the player is out of the league. Sorry Wes Welker, Greg Jennings, Sebastian Vollmer, Osi Umenyiora, Dwight Freeney, Charles Woodson, Ronde Barber and Brian Urlacher, to name a few.

Running backs? No thanks

Unless Adrian Peterson hits the market, skip running backs with high mileage. There are too many good, young, cheap models available to go spending money on a used one. Sorry Ahmad Bradshaw, Reggie Bush, Felix Jones, Steven Jackson and slippery little LaRod Stephens-Howling.

Injuries? No way, no how

A doctor can clear a guy, but if the guy's body is breaking down, it's breaking down. Jake Long is 28 and might be the best left tackle of his generation. But are you willing to pay more than $10 million a year to a man who might be heading down the same path that Tony Boselli traveled? Sorry Jake, Brent Grimes, Danny Amendola, Fred Davis and Kenny Phillips.

Reggie Bush

Former first-rounders? Beware

Watch out for household names. If a former first-round draft pick is on this list in his 20s, he's either a major injury risk, unreliable or he's been a flop for the past four years. Take on a resurrection project only when it's financially easy to bail when it fails. Sorry Sedrick Ellis, Glenn Dorsey and Dominique Rodgers-Cromartie.

Seven to select

Keenan Lewis, CB, Steelers

Pittsburgh not only stands to lose the best receiver and overall player available (Mike Wallace), it's also in danger of losing the best cornerback. He's a big, confident corner on the rise. And he's only 26.

Connor Barwin, OLB, Texans

His sack total dipped from 11.5 in 2011 to three last year. But he played 94 percent of Houston's defensive snaps. He also switched positions during the season. A target for 3-4 teams. He's also only 26.

Jared Cook, TE, Titans

A 6-4, 235-pounder with the athleticism of a big receiver. He had a career-high four TDs last year and doesn't turn 26 until April. Could become a star in a better offense with a better quarterback.

Rey Maualuga, MLB, Bengals

In a market virtually devoid of quality inside linebackers stands a special 6-1, 254-pounder with 59 starts at age 26.

Cliff Avril, DE, Lions

He's not the most complete player, but he's the kind of good, young pass rusher that rarely hits the open market. At 26, he also has plenty of time to become a great pass rusher..

Louis Vasquez, RG, Chargers

He turns only 26 in April. A 6-4, 330-pounder with 54 starts in 54 games.

Sean Smith, CB, Dolphins

He's not the most consistent player, but he's 25 years old and plays cornerback at 6-3, 218 pounds.

MARK CRAIG