Trade deadlines have already passed in many fantasy leagues, so I'll focus more on lesser-owned players to assess their value, rather than potential buy-low or sell-high candidates. If your league doesn't cut off trading in August, remember that it's never too late to take advantage of a desperate owner trying to stay out of the basement.

Scouting players who may be undervalued and re-assessing players who may be overvalued is a weekly exercise that you should pursue. Fortunately, I have you covered. Here are a few players who are good buy or sell candidates based on their current performance compared to ownership levels. This could mean scooping them off waivers if possible or actively seeking a trade to acquire or discard certain players in order to maximize value.

I will include one player at each key position group (Infield, Outfield, Pitcher). Ownership levels are taken from Yahoo! Fantasy Sports.

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Underrated Players - Week 21

Cory Spangenberg (2B/3B, SD) 36% owned

Believe it or not, Spangenberg has been one of the best hitters in all of baseball throughout the second half of the season. He's batting .322 with seven HR, 23 RBI, and 27 R over the last 32 games. This former first-round pick is finally playing up to expectations and seems to have found a comfort zone at third base. If there's one caveat, it's his weakness against southpaws; he's hitting .217 with a 2:38 BB:K in 83 at-bats versus LHP. Even if the Padres aren't exactly raking these days, Spangenberg has found a way to consistently produce at the top of that lineup.

Byron Buxton (OF, MIN) 30% owned

I just can't quit you, Byron. The last two weeks have gotten Buxton believers all giddy again, as he's ripping the cover off the ball. Buxton is batting .314 with two homers, nine RBI and five steals in the last two weeks and has only gone hitless twice in August. There's been a modest drop in his strikeout rate as well, giving hope to the idea that he's suddenly figured it all out. Perhaps he suddenly realized the season is almost over and he better do something quick if he wants to keep his starting job next year. Either way, Buxton is too talented to ignore when he's on fire like he is. At least consider streaming him if he's available.

Luis Castillo (SP, CIN) 31% owned

Trusting rookie pitchers is always a dangerous proposition. Trusting one who pitches half his games in Great American Ball Park in Cincinnati is borderline Austin Powers territory. So far so good for Castillo, who has tossed four straight quality starts and managed to blank the Cubs for six innings the other day. He currently holds a 3.39 ERA, 1.18 WHIP, and is striking out a batter per inning. Be warned that he had less than 100 innings of experience above Single-A level before his call-up, he only converted to a starter midway through 2015, and he's already surpassed his career high for innings pitched in a season. That said, go out and grab him while he's hot! If he can keep inducing ground balls at a 56% clip while striking out 24% of batters faced, he'll be just fine for another few starts.

Overrated Players - Week 21

Yoan Moncada (2B/3B, CHW) 43% owned

He could very well get on a roll and prove me completely wrong, but so far it looks like the rookie still has some developing to do. Moncada was rightly added in nearly half of all Yahoo leagues after getting the call in late July. He may be best left on the bench or thrown back on waivers if your league isn't deep enough. Moncada is slashing .182/.327/.341 and has just three multi-hit games in 25 contests. A 36.7% strikeout rate and 14.6% swinging strike rate show that he may need another offseason before he can try to reach the heights of Cody Bellinger or Aaron Judge.

Brett Gardner (OF, NYY) 75% owned

Tempting as it is to put Gregory Polanco in this space again, let's focus on a player on a team that has struggled to recapture its first-half magic. Gardner reached a career-high 19 HR already, although only four have come in the second half. He hasn't stolen a base since the first of August either. Needless to say, with Aaron Judge and company struggling, he isn't scoring as many runs either. Gardner is certainly a fine third or fourth outfielder in leagues of 12 or more teams, but you had to know he wouldn't maintain his early pace all year.

Jason Vargas (SP, KC) 85% owned

Someone else who's lost that lovin' feeling since the All-Star break is one of KC's representatives in the game, starter Jason Vargas. If you were slow to warm up to him after his shockingly strong start, you weren't alone. Vargas dominated in April with a 1.40 ERA, .221 BAA and 28:2 K:BB. The last 30 days have been a little different. Vargas holds a 6.44 ERA, .306 BAA and 24:14 K:BB since the break. While he has posted wins and quality starts in two of his last four outings, his bad starts have been really ugly. Plus, his last win came against the White Sox, which shouldn't even count. Vargas isn't someone you want to depend on if you are neck-and-neck in the ERA race with leaguemates down the stretch.

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