Well, it is not our fault that we have more knowledge about the used-hardware market. Dont feel bad about it.
Did a swap 4-5 years ago, 780ti + apprx. 100€ for a 980ti.
I as well felt like I was cheating the guy. He claimed the 980ti gave him bad FPS in csgo. We even tested and funny enough, he had more frames with the 780ti.
Benchmarks etc showed the 980ti outperforming the 780ti by quite a margin, but he was only focused on performance in csgo. Lucky for me.
We made a deal, that if he would have second thoughts within the next 2 weeks, he could contact me. He never did and i'm still rocking that card, haha.