These two tips will work when the seller is willing to negotiate the price. Remember prices are always negotiable, even if they have a price tag on it.
Figure out how much the item probably cost the seller to initially buy it for and then add whatever amount you think is a reasonable profit for the seller to make off you and then give the seller a number that is below your reasonable amount.
The figure that you state to the seller that is below your reasonable amount may be accepted, if it is not, tell them that you have no more money and that is the lowest you can buy for. Now, if the seller still doesn’t budge, then tell them that you have no more money in your pocket - proceed to check your pockets and find nothing. If the seller still doesn’t budge on the price, then check your back pocket and act as if you just found some more money, and tell the seller that’s all you have. At this point, the seller will agree, and you have paid the seller the amount that you initially wanted in the first place.