For Mixers and Exchanges, having a Letter of Guarantee (LoG) feature is a good way to provide digital proof of a trade. If there is an issue or an unexpected problem with a transaction, the LoG can help identify if the trade indeed happened or not. A LoG usually contains information about the deposit and withdrawal, including the value of the assets involved.
A LoG can usually be verified by anyone online, as long as they have access to the project's public key.
At WeLive Crypto, if a project supports a verifiable LoG feature, we ask the listing owner to provide either the Bitcoin address or the PGP public key used to verify the LoG. This allows our users to check the authenticity of the LoG they receive when they initiate a trade.