Short answer:stock. The simplest form of resale provides value to the guys on the big end by relieving them of the pain of dealing with many small transactions and to the small guys by keeping stock(#) and letting them buy smaller quantities of stuff.

(#) Which is obviously a risk - who knows if you find buyers for all of the 100 thingumajigs you had bought wholesale. And taking this risk is a part of the value provided by the reseller.