Lenses I sold:
70-300mm f/4-5.6 non-L - I bought it cheap @ 369, ended up about an even trade for the optically superior EF-S 55-250 STM. As of now I use the 70-200 f/2.8 IS II USM for FF and the 55-250 STM for my crop. I would replace the EF-S 55-250 with a high quality compact/light FF lens in a second, but Canon does not currently make one.
17-55 f/2.8 IS USM - When I bought a full frame camera, I sold this lens for two reasons despite also having a crop camera.
First, most of the range can be serviced by the FF 24-70 as I rarely used 17-24mm on crop - most shots I took with it on crop were between 28mm and 55mm (normal to tele).
Second, though I got some great shots with it, I was never totally satisfied. I can't really put my finger on it, but the a lot of the pics seem to have an almost dated look to them when compared to some of the newer primes and zooms. Maybe a bit soft, or the color a bit muted? Either way, though this is the best lens for crop currently I was never really 100% satisifed with it optically; maybe we will see a update of it soon.
EF 135mm f/2.8 Softfocus - I was interested in this lens for the softfocus effect... I really like to use optical effects over digital when possible. But, despite much experimentation I was not happy with the softfocus effect it delivered. I couldn't shoot wide open without extreme/unusable softfocus even at the dial at 1/4 of max, plus even when stopped down I got better results out of software with less hassle. The ancient AF system did not help, though it is a bargain pricewise.
Lenses I was tempted to sell but did not
EF-S 10-22mm f/3.5-4.5 - When I got a FF I was tempted to swap this out for the 16-35mm f/2.8, but after looking at samples I am not convinced the 16-35 is better - may actually be worse. The 10-22 seems to have MUCH less barrel distortion, so much less that I'd say it makes up for the lack of f/2.8. I would love to see a 16-35 f/2.8 with more controlled distortion but there is a good chance this is simply an advantage of crop for this type of lens. I think I am going to keep this one for a while, one of the best shots I ever took was done with this lens.
EF 40mm - I actually sold this once, rebought it because I kinda missed it a bit. It is not very useful on crop but is a fun lens on FF. I might sell again if the 50mm IS f/1.8 comes out soon and is compact, who knows
