S.J.
In order to require immediate payment, you have to make the selection titled "require immediate payment" when listing the item. You can only require immediate payment under "buy it now" listings I believe. If you list something as an auction, you run the risk of the person taking days to pay, or never paying. This is one of the major drawbacks of ebay.
What I do is this: In my auction listing, I make VERY CLEAR the item must be paid for within 4 days of the auction's end or I will relist and I will file a non paying bidder claim. That way, people cannot say they didn't know. You choose how long you are willing to wait, some people wait longer than 4 days, some less.
ETA: I am not sure what Christy is referring to, but a buyer doesn't need to leave feedback in order to pay you. I pay all the time without leaving feedback and vice versa. They simply need to log in to pay pal/ebay and pay!!
ETA: Because you didn't select "require immediate payment" when you listed the buy it now listing. You have to select that. Unless you do, someone could still do a "buy it now" on your listing and never pay......