in which case i would be saying to them something along the lines below.
“im really sorry your carpet has been marked and i can understand where you are coming from however i believe asking for a full carpet replacement in this case is unfair owing to the age and condition of the carpet and the fact that i moved the pc at your own request. As you are aware i have tried to clean it myself and couldn’t. What i can do is offer you £xx off the pc OR request a professional cleaning service for the portion of affected carpet at my cost. However i won’t be replacing the full carpet or having the entire carpet professionally cleaned as that is not a proportionate or reasonable response to the incident.”
the £xx i would suggest being less than the cost of a cleaner as they are more likely to want cash than a clean carpet. And try to ensure that it doesn’t cut all of your profit away.