I have a nifty fifty and am debating selling... or should I keep it? The other lenses I have in my bag (which all fit perfectly): Canon 135L f/2, Canon 85 f/1.8, Tamron 28 - 75 f/2.8, and Tokina 16-28 f/2.8. I feel as though the Tamron is a sharper lens than the 50mm, and it covers that range. Will I regret selling it? Part of me says, "keep it... it was a good deal and you won't make much on it anyway" (paid $84 for it new in 2009) and the other part of me says, "you'll never use it now that you have the 85 and the 135, which are better portrait lenses anyway".
Shooting with a 5D Mark II, if it matters. I do 90% portrait photography, mixed in with some landscape and macro (flowers and stuff).
Keep it ... it will serve as a back up lens ... also, as you said that by selling it "you won't make much on it anyway" ... besides if/when you want to do some video in low light (at that focal length) it will come in really handy. Over a period of 6 years I had three Canon 50 f/1.8 lenses, the only reason I don't have any of them now is coz I gave them away as a "free gift bundle" to "enhance" the sale of my Canon 400D, 500D & 7D. So keep it, if for nothing else one day it will come in handy when you want to sell some of your other gear.
5D MK III, 16-35/2.8 L II, 24-70/2.8 VC, 24-105/4 L IS, 70-200/2.8 L IS II, 100/2.8 L IS, 40/2.8, 50/1.4, 85/1.8, 150-500 OS, 600EX-RTx2, ST-E3-RT, Rokinon 24/3.5 T-S, D7100+18-300 VR, YN568EX, 055CXPRO4+468MGRC2, 190CXPRO4+322RC2, Gorillapod Focus+496RC2, CamRanger, B+W filters, Konova K3/31.5 etc