People send me money to my bank account, I send them their coins. What kind of explanation should I provide to these questions to ensure the sustainability of my account? Especially for the first question; we do not have any contracts, and the only documentation I can get is the invoices from the exchange. Should I be completely open with them or should I try to hide that I am selling cryptocurrency? Maybe there are someone who have had similar experience?
Edited: the country I am located in, crypto is completely legal.
If crypto is legal in your country then what is wrong in telling bank. Just tell them you are engaged in trading of cryptocurrencies. If they are ok with it then no problem but if they raise questions and try to limit your account then you can always shift to other banks. In my country, public sector banks owned by government are bit conservative in respect of foreign remittances so my bank limited my account stating that I can no longer receive funds from outside my country. Then I shifted to private sector bank and everything running smoothly since then.
But do check that you pay taxes on your incomes else you can land in big trouble.