I don't think it's legal. It's really important to be careful when considering buying software licenses from third-party websites that offer significantly lower prices than the official ones.
Sometimes, these websites could be selling licenses that were obtained illegally or without permission, which could get you into legal trouble. There's also the risk of running into issues with the software itself, like getting incomplete or invalid licenses.
To play it safe, it's always best to buy software licenses directly from the official vendors, like cPanel or Plesk. This way, you can be sure you're getting a legitimate and legal product. While the cheaper prices might seem appealing, it's not worth the potential problems that could come with it.