Many items are priced sharply lower in quantity. Buying large quantities many permit you to deal with a big distributor or direct with a factory. Suppose you buy an item in quantity, then break it down and resell at a discount to others. Maybe caulk is $1.39 a tube. You buy a palette load for $.50 a tube and sell the remainders to others at $.79.
Another friend buys items by the case from one website, and then sells them individually on eBay
for a profit. You could buy and split up a big load of, say, lumber to three or more people. In other words, buy with friends. You could advertise for the friends. Or you could buy for yourself and divide later by advertising the items in the newspaper or on the Internet.