I don't get it. Why does it need to sell the ETH first? Why don't it just buy high and sell low (ideally) ignoring what's deposited in my wallet? Doesn't the cycle start with the buy?
What if I don't want Gunbot to sell what I have in wallet? Please, explain to me how the trading strategy, that requires to deplete existing funds first, works.
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Gunbot will always sell all assets when selling, and will buy according to your set trading limit." From the wiki. That's how the bot is built, it can't ignore deposited assets.
So it needs a bought price for an existing asset (and it will sell everything in the immediate next sale if you manually buy some more of the asset during a trade). And, the moment it is provided with a bought price, it will look to sell the account first, with two exceptions: if you have activated double up and the asset has fallen in value in regard to the provided bought price, then it will buy more of the asset (if within set parameters of double up) OR if sell enabled is set to false. If you have reversal trading activated it will also immediately sell (if within set parameters of reversal trading).
You could circumvent this issue for a small sum of an asset with the min volume to buy and min volume to sell settings, so a small amount of your asset can be kept in your account. But I don't know how much of ETH you intend of keeping, so this may or may not be practical for your specific issue, as these settings are meant more to make the bot ignore the dust in your asset wallet(s).
If you want to keep a coin which is not your base currency, you best move it to a separate account if your exchange allows multiple accounts (some do) - or don't trade it. There might by a third party plugin which enables what you wish (I don't use any, so any help from other users would be very welcome here), but not the bot's original features.
You could keep ETH in your wallet if ETH was your base currency (there's a bot setting on how much of base you keep as your reserve). In your described case you are trading ETH against your base BTC, in which case you can keep only BTC in reserve.