Is this the GAS group meeting?
EF-s 18-55 III non IS was my starter lens. I got this with a 1000D and learned to use the camera with that.
Sold to get a 50 1.8 II as my budget was limited back then, and I liked the bokeh pictures i could make.
Sold the 50 1.8 II to buy a zoom again. The 17-85mm
Replaced that with a 28 1.8 and a 50 1.4 Added a 70-300 IS (non L)
Sold the lot to get a 24-70 2.8
bought a 100-400, and sold it to get a 70-200 2.8 IS with 2x TC, sold the TC to get a 1.4 which I never got around to.
Bought a 10-22 from canon, sold it to buy a Lotus Europa (yes, a car, yes it was for the same amount of money)
Bought a 18-55 IS II for filming, gave that to a friend
Bought a tokina 11-16, upgraded to full frame body so had no use for it.
Sold the lotus, bought a 15mm fisheye 45 minutes later, hated autofocus, traded it for a 17-40L which I sold to buy a 8-15mm fisheye.
And then there is the gear I bought because it was cheap, and sold on for a profit. Which is another 17-85, another 70-300 IS. A 28-80, a 55-200, a 18-200 IS, a 50 1.8 first version, sigma 150-500, tamron 70-300 with macro (junk) kenko 1.5TC (also junk) All were sold after only testing them, and for a profit.
I have never sold a lens I've bought new, and therefore I've never sold at a loss.
Right now I have my 40mm for sale (buying a 50 1.4 again), which will be my first shop purchased lens to sell. But I bought at a rebate, so loss should be minimal
To recap reason. I've sold the 18-55 non IS, 50 1.8 II and 17-85 because they were rubbish. All other lenses I sold to get something I liked more. The 15mm fisheye didnt focus very well on my body, which I think was a bad copy or something.
If I could afford it I would still have the 28 1.8, 50 1.4 and 100-400. But like most people, there are limits to what I can spend. So I have whats in my signature.