I have had to ban two people this week.
I have put this out before. If I find you are buying gacha items from my yardsale, and then reselling them, especially at an insanely marked up rate, I will ban you from the store parcel and my vendors.
I sell off my extras so other people can enjoy the items I cannot use. I don't go to other folks' in world locations or Marketplace stores and buy items that I can pass on to others for a profit, and don't feel it is fair for someone else to do that to me.
I sell all of my Commons at or below pull price because I think it is fair. Yes, like most other folks, I do charge extra for Rares, but I don't think my prices on those are completely unreasonable given what I see other folks charging.
If you cannot respect this simple rule, then you will simply have to find another place to shop for your pre-purchased/pre-won/pre-pulled gacha goodies.