I'm not an expert on the talent of pitchers on other teams, but here are some pitchers who are off to a pretty good start, this year, and are on teams that may be sellers this summer:
Pitcher- Team- WHIP- K/9
Alex Colome CWS 0.55/8.3
Will Smith SFG 0.77/13.1
Shane Greene DET 0.83/9.8
Trevor Gott SFG 0.83/9.4
Felix Pena LAA 0.84/9.4
Blaine Hardy DET 0.91/3.9
Jake Diekman KCR 0.91/13.3
Sam Dyson SFG 0.92/9.0
Ken Giles TOR 0.97/14.1
Hansel Robles LAA 0.99/9.3
Sam Gaviglio TOR 1.00/8.8
R. Stephenson CIN 1.00/12.6
Joe Biagini TOR 1.07/8.1
Ty Buttrey LAA 1.08/11.4 (Top 10 in RP'er fWAR)
Possible sellers this summer:
AZ: Yoan Lopez 0.81/6.0
CLE: Brad Hand 0.91/13.5
NYM: Seth Lugo 1.00/11.8, Edwin Diaz 1.05/14.3
Note: this list is based soley on the top WHIP RP'ers with 20+ IP this year. I'm not saying any of these guys are a good fit or can be acquired.