Stock product before first Sale?

In theory,

Assume for the moment that I were to try to sell paper cups.

How can I approach selling them?

Do I actually go out and buy 5,000 cups and then sell them? Or should I stock nothing and hope that someone will buy them? Or should I be in the middle with a few hundred?

Should I look for a manufacturer of these cups first or after I try to make sales?

Just curious if anyone knows of a good book or other resources on this topic.