for your first question, the answer is yes, that’s stealing. justice demands that no one suffer loss in an exchange.
the second part, haggling. it is moral, if that is the system. if the price is set by striking the bargain on the spot, the seller need not say yes to anything. if he agrees, you have to presume he is getting what he needs. even if you can pay more, you are not obliged to. you just have to be fair.
if that is not the system, and there is a price, but you coerce the person into selling the object for less, that is unjust. you don’t have to buy the item, and you are not entitled to it. this is especially true if you can in fact pay for it at it’s real price. (if it is a matter of human dignity, this doesn’t apply. everyone is entitled to the means of life. if the seller unjustly withholds the means of decent human living by over pricing goods, it is not unjust to force the price down.) if the seller is despirate and figures if he doesn’t take less, he/she will get nothing, but the buyer cannot pay the full price, the situation could be seen as becoming de facto a haggling system. this of course is less than ideal and not a maxim. it would only apply to a real situation. justice is always the issue and guide.