the more you deposit, the less you lose on fees for account replenishment

friends, can you tell me how to trade? For now, I am just exchanging for USDT and back. Why can I buy a couple of coins on the exchange, but when selling it says a minimum of 5-15? Is there a possibility to set up automatic trading? Why can't I immediately deposit my own money into the balance, but only through someone else's card, depositing 1200 UAH, and only 1100 UAH got credited.