You're ok. When I buy used equipment, I ALWAYS assume someone has done some work under the hood. And when I sell something, I will tell buyers one of two things... (1) "I bought this, have not pulled the chassis, I don't know if anyone has made repairs or modded the amp, so to the best of my ability, I am stating it is in good working order and may not be 100% original" .... or (2) "I bought this amp, needed to make some repairs, I tried to use component values that are close to the originals, the amp is in good working order but not 100% original."
You cannot be responsible for something you do not know. So just tell the story that reflects what you know.