They can give me Negative feedback if they are happy then, i know my mind is clean because i will tell the costumers that the keys may be revoked so basicly it's not a scam.
Example :
I saw someone selling 1 ebook here how to get unlimited spotify and AUTORENEW accounts (and thats for sure a scam/lie) and he did not said that this "trick" may end in the future.
For the thats is a scam.
No body can guarantee for a method how long will it work, it can be patched by websites dev, but if you start selling you may mention that if the key will stop working in "1 month" then i will replace with another.
At least guarantee an amount of time. I would suggest to contact maybe mods to get an opinion from them.