From the firearm blog: #7 – Try to buy a used firearm a customer is trying to sell to the gun shop – You wouldn’t try to undercut a car salesman when he’s being traded a car so how do you think it’s socially acceptable to do it in a gun store?… Unspoken Rule of Thumb: if the customer and gun store can’t come to an agreement, ask the individual once they leave the store. It’s incredible tacky and rude to make an offer while the trade is in process.