I strongly advise against attempting to buy stolen BTC or any other illegal activity. Not only is it morally wrong, but it can also result in serious legal consequences. In addition, there is no guarantee that you will actually receive the BTC you are paying for, as you have already experienced in your previous attempt. Seriously, how could you think that paying only $50 for a wallet containing $500 worth of BTC was a legitimate deal? It simply doesn't make sense for anyone to sell BTC at such a steep discount, whether the coins was acquired through illegal means or not.
Instead, I recommend that you invest in BTC through legitimate means, such as buying from a reputable crypto exchange or from a reputable seller if you want to go the P2P route. These options may not provide the discounts that you are looking for, but they are safe and legal ways to invest in BTC.