Sigma 120-300 is loosening

Hello everyone,
I bought a Sigma 120-300mm F/2.8 DG OS SPORTS HSM Canon in 2015, haven't used it a lot the last 2 years so i decided that i want to sell it. When i removed it from the pelicase where i stored it in i found out the lens is a bit loose, I don't think that it's normal with this lens or what the issue is. I made a video that is showing the issue, it's a moving like 3mm. The lens is focussing and working like it should do though.
Does anyone know what this issue is? Can i solve it myself or do i need to send it for repair to Sigma, and what wil be the repair cost be for something like this?
Video:
And 2 photo's where you can see the difference:


Thank you for your help and answers!
I bought a Sigma 120-300mm F/2.8 DG OS SPORTS HSM Canon in 2015, haven't used it a lot the last 2 years so i decided that i want to sell it. When i removed it from the pelicase where i stored it in i found out the lens is a bit loose, I don't think that it's normal with this lens or what the issue is. I made a video that is showing the issue, it's a moving like 3mm. The lens is focussing and working like it should do though.
Does anyone know what this issue is? Can i solve it myself or do i need to send it for repair to Sigma, and what wil be the repair cost be for something like this?
Video:
And 2 photo's where you can see the difference:


Thank you for your help and answers!