I've seen a lot of Yoona's hatred still ongoing, but in her defense, she didn't just buy an expensive item (and the price dropped) from Seyeong. Heejae went around asking Yoona about Seyeong and acted like he got scammed.
If I was Yoona, I'd be mad too. Her bullying him in the clan IN GAME doesn't seem too evil now. Plus, she never doxxed him (aside from telling Heejae) not did she take this into real life.
When she met Seyeong the first time, despite her suspicion, she did nothing. Second time, just staring and said a few lines.
Now what's concerning is when she said she'd put her prize to good use. If she's using it on Seyeong, I'd be mad. If it's on Heejae, mofo deserves that lol.