Don't listen to anyone who tells you it's fine to send/receive funds directly between Coinbase and Books.
It might be tougher to detect these days with everyone using a different wallet every transaction... but as soon as 1 user confirms any wallet address connected to a book, Coinbase then follow those coins to the point they are cashed out or conglomerated into a storage wallet, then reverse track every other wallet that has sent coins there.
If any of those now thousands of connected wallets are found to be have been funded directly from a Coinbase account, you lose your account.
Here's their terms about it;
APPENDIX 1: PROHIBITED USE, PROHIBITED BUSINESSES AND CONDITIONAL USE
Prohibited Use
You may not use your Coinbase Account to engage in the following categories of activity ("Prohibited Uses"). The specific types of use listed below are representative, but not exhaustive. If you are uncertain as to whether or not your use of Coinbase Services involves a Prohibited Use, or have questions about how these requirements apply to you, please submit a request at
https://support.coinbase.com. By opening a Coinbase Account, you confirm that you will not use your Account to do any of the following:
- Gambling: Lotteries; bidding fee auctions; sports forecasting or odds making; fantasy sports leagues with cash prizes; internet gaming; contests; sweepstakes; games of chance
https://www.coinbase.com/legal/user_agreement?locale=us