So Lecker

Somebody bought a very expensive item from my MP shop and in the same transaction of 9 others. One day later the L$ where deleted by LL, because they where not void. The official explanation was as follows: "Our investigation has determined that the L$ sent to you were obtained through unauthorized and fraudulent activity within Second Life" My goods are gone (it was a unique nocopy gacha) and I have no idea how I can protect myself against that in the future. Is there a chance to block buyers? Do you have any experience with such fraud? Does that happen often? I lost a bit the trust in the marketplace...