According to badeballtradevalues.com, these are the SP'ers with the worst negative value:
(Some are not on the trading block. Some are very good pitchers who are owed a lot of money.)
-112 Strasburg
-59 Cole
-50 Bumgarner
-45 Corbin
-33 Verlander
-28 Mikolas
-21 Cueto
-17 Greinke
-18 Sale
-12 Cobb
-11 Eovaldi
-11 Price
-10 Roark
-9 Duffy
-8 Lyles & Keuchel
-4 Miley
-3 Kikuchi, Gibson & Ryu
RH RP'er
-15 Jansen
-13 Familia
-11 Kimbrell & Kelly
-6 Martinez
-5 Ottavino & Harris
LH RP'er
-9 Miller & Britton
-8 Smith
-4 Chapman & Pomeranz