But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil. Matthew 5:37
The following quote is a bit out of date, but still represents a general attitude in the Middle East. Those folks generally love to haggle.
"As fixed prices are unknown in the East, buying and selling is a complicated business. Each merchant has his own prices and quotes them to each customer as he pleases. The customer is afraid of being cheated and has his own idea of the price. This makes business burdensome.
For instance, if one decides to buy a pair of shoes or a garment, it means a day’s work and much exhaustion because of mistrust and incessant bargaining before making the purchase. When the price cannot be settled by bargaining, merchants and their customers generally take oaths by temples and holy names in proof of their sincerity. They take an oath saying, "By God’s name and his holy angels, this pair of shoes cost me six dollars but you can have it for three dollars". When such oaths are ineffective then they resort to swearing. Thus, "If I lie to you I am the son of a dog or an ass, the shoes cost me three dollars but I will let you have them for a dollar and a half." To all of this the suspicious customer replies, "By my only son’s head, I will not pay you more than a dollar.” If this fails, the merchant is apt to spit in the face of the customer.
Instead of this conventional cross play, waste of time and temper, Jesus here insists on directness and frankness in dealing with one another. He knew that when a man is cheated he would try to cheat others, and if one is deceived by oaths uttered on sacred names he would likewise do the same in transactions with others. "Yes, yes" and "No, no" is the only successful and straightforward method in business. The Orient is only now learning that such a method is far superior to their traditional system."
(Lamsa, George M. 1936. Gospel Light. San Francisco: Harper Collins., pgs 39-40)