With the increasing popularity of blockchains, cryptocurrencies are now accepted for the purchase of digital goods, such as e-books or gift cards. A contingent payment is a cryptographic protocol that models digital purchases, and it involves a buyer and a seller. The buyer owns crypto-coins, and the seller owns a digital product. Contingent payment ensures that the buyer and the seller can exchange coins and product securely. However, observers of the blockchain might learn which buyer purchased from which seller based on the information contained in the transaction. Is it possible to extend contingent payment so that the relationship between buyer and seller is hidden? In this talk, I will present how contingent payment works, as well as coin mixing, practical technique to hide the relationship between a sender and a receiver in a transaction regardless of the blockchain. Then, I will show that existing coin mixing schemes cannot be applied to contingent payment as they lead to devastating attacks. My presentation ends with MixBuy, the first protocol that hides the relationship between buyer and seller in a contingent payment, regardless of the blockchain. This talk is related to the paper: https://eprint.iacr.org/2024/953, which will be presented at The 25th Privacy Enhancing Technologies Symposium in 2025.