Not even slightly suspicious that someone who joined *checks* 2 hrs ago wants to buy the loss machine.
But, giving benefit of the doubt, it is a bad idea to buy plots made with someone else’s key, and an even worse idea to buy what you know is less than all of the plots made on someone else’s key - if you trust the person you’re buying plots from, fair enough take the risk, but if you believe that person is potentially selling plots made with that key to other people, you’d have to trust all of the (unknown) people that person is selling the plots to as well - you’d all have access to the same key.
If you want cloud plots, pretty much any plotting service will make plots for you with your own key for this price or less, and without the AWS egress fee.
If you want to make your own massive recurring financial liability - find out which instance class in AWS has the best specs for the current spot price, rent loads of them and plot away. Individual instance specs barely matter if you rent enough. The difficult bit will not be making plots, it will be getting them out of AWS.