- all sites (services) keep logs such as your IP addresses, your deposits, withdrawals (the transaction IDs) because they use them for many purposes like for example if you wanted to recover your account if you lose your password.
- using these services to mix coins means you only want "public" to not know about your other addresses but the site you use and in case the law can find out about it easily.
- as for documents i know btc-e doesn't ask for any, i also use bittrex and they never asked me for documents but i think for very large volumes they ask for it. and they are both legit places and never hold bitcoin hostage
Let's compare some usage aspects of the services from the first and second group. (groups from my first post in this topic)
1. Technically speaking, the results achieved are completely the same - Payment goes to the address A, Payment is received from the address B not associated with the first. This fact can be easily verified. (When using the services of the second group, in rare cases, the desired result may not be achieved from the first exchange. However, it is easily achieved by repeating the operation).
2. After the exchange (mix), the user activity log is an obvious possibility of reliably restoring the connection between addresses A and B.
I agree with you that all sites that are suitable for the mix (exchangers, gaming sites and other similar sites) are likely to keep logs. It is also most likely that mix services keep logs. They may have different reasons of keeping logs. Some may be keeping logs "just in case", while others may be doing to be able to thoroughly deal with issues when the need arises, for example, when the service is hacked. An extreme option in this sense can be the mixer, which is created by intelligence agencies to gather information. So I would proceed on the basis that there is a log in either case. Another issue is understanding to what extent is the log accessible to interested parties. I would have divided interested parties into two categories: law enforcement agencies and others.
For the "others" category, logs will be equally inaccessible in a case where a special mixer and a site from the second group are used.
For the law enforcement agencies logs will be accessible in a case of using "second group sites" and will be unreachable in case of a special mixer (if mixer is not honeypot). Thus, we can conclude:
If you wish to hide from police etc - you'd better use special mixer. In other cases, sites from second group may fit your needs.