If the seller approves your quotation, then you can buy it according to your quotation. If the seller doesn't want to, he can give you a new offer or reject your offer. This price is reserved for you for 24 hours, unless someone buys it within 24 hours, then you have no chance.
When you enter the trading counter and search for your favorite goods, you can see that some goods are marked with the word "return" and you can make a counter-offer, but those without the word "return" can't.