M1 iMac dust behind screen no access to clean

my m1 iMac 8core is 3 years 3months old in perfect condition not a scratch on it carefully looked after in non smoking home yet when I decided to trade it in they said it had dust behind the screen so they didn’t want it i didn’t worry as I remembered seeing u tube tutorials on how to clean behind the screen of the original iMacs so I thought this would be similar how wrong I am its a sealed unit so apple what am I supposed to do with my iMac throw it away over £1600 down the toilet or do I bring it to you to fix