Earlier today I sold my 24-70 / 4L IS at a good price. The idea was that I would buy a 17-40 / 4L or 16-35mm f / 4L. But it changed direction quickly.
In 2014 I bought my most expensive lenses ever. A sigma 120-300 / 2.8 OS. I was very happy with the sharpness and everything. The problem was the weight and the focal length. 300 mm was shorter when I got to the full format. The back began to hurt when I was overweight. Eventually, I sold it and bought a Tamron 150-600 that I'm happy with. My full-frame cameras handle higher ISO good and when it is possible to shoot with a larger aperture.
Now a year later, I have thought about it and realized that my biggest loss of all sales and exchanges, Sigma 120-300 / 2.8 OS. Today I am in better shape than previously and many shooting situations (from the hide out in the forest) requires fast aperture. The first time I photograph the eagles were in 2014. With Sigma. I made it without problems in low-light conditions with the 2.8 - 3.2 and ISO 1600. Selected photos from this trip. Earlier this year, tested the Tamron in the same lighting conditions and distances, and the same camera. I had to be at f / 7.1 -> f / 10 for good sharpness and ISO 4000 to get the same shutter speed. It was completely different images.
I usually buy from a store with used market and have always been pleased. They currently have the first version of the 120-300 / 2.8 without image stabilizer and with inferior picture quality. The price is 1000 USD.
During the afternoon, 120-300 / 2.8 OS is in stock. Complete with everything. The condition was grade B (a-b-c). The price was 940 USD. I took it right away. There is another shop selling a used example, but for 1600 USD. My idea is to have this lens for all situations that require a similar lens. My 150-600 will still be used the most, if the weather allows it. On Wednesday, I get the answer to how 120-300 / 2.8 OS is and looks like. More information to come. Hoping for a good copy.
My old 28-90 / 4.5-5.6 II may help me in situations that require a normal zoom for a while.
but I trust this store because I've bought used lenses from them many times and they don't lie about the condition. 
