Basically this topic is relative to the purpose of advertising. Do you want someone coming into your topic and marking down your item while you take the time to bump it? I think not.
In examples 1 and 2, Person A is the topic creator.
EX1. Person A is selling an object for 30 dollars. Person B decides to come into the topic and says "I'm selling the item for 25 dollars, send me a PM."
EX2. Person A is buying something for 30 dollars. Person B comes in and says "I'll pay 35 dollars for that same item."