It mean's you have to buy the whole case, or 2 case's, or whatever amount they sell in each set or box. Like buying bulk at Sam's Club, when you buy egg's you have to by 5 dozen at once, because they only come 5 dozen to a sealed container. If you buy shredded cheese you have to buy a 5 lb bag, because it's only sold in large quantities.
When a merchant is selling clothes in bulk or by lot, he/she is typically selling at a wholesale level in large quantities. In other words, these venders do not sell individual items or accommodate personal orders. In any case, you should contact the seller to find out about his/her policy.