To sell a part of my Bitcoin/Ethereum balance, in your advertisement, you can set "Total BTC/ETH amount" as following:
You can also do following methods:
Activate 2-Step Authentication (if you have not already done so, you can see how to enable two-step authentication here).
Go to "BTC/ETH Wallet", then create a withdrawal request to any of your personal addresses (but do not confirm Authy code to approve your request) with the amount you want to keep. As such, the balance in the withdrawal request has been frozen, and you can only sell up to your available Bitcoin/Ethereum. Remember to turn off your ads after you have sold your desired amount, when you want to re-use your frozen balance, just go to the history of withdrawal request, then cancel the request. Example: You have 10 BTCs, you want to sell 1 BTC only, you create withdrawal request with 9 BTC but do not confirm.