You can see it clearly in the OP’s for sale thread pics so it isn’t exactly hidden, nor is it anything major IMO. Nothing hidden by the OP, just looks like the buyer changed their mind at home.
Seems to me that the “buyer” is better off without the bike due to him being blind as a bat and presumably without any feeling in his fingers.
If he spent 30mins inspecting it and couldn’t see that was dented then what on earth was he doing?
It’s got all the hallmarks of the buyer failing to get it past his misses 😀 and how the hell you can spend 30mins inspecting a MTB without taking it apart and putting it back together, is beyond me…..?