Only the seller can cancel an order. The most a buyer can do is request the seller to cancel at your request or make a certain kind of request in the first hour (in My eBay: Purchase History in the actions for that order) which the seller has 3 days to respond to (but since you already paid there happens if s/he doesn't respond).
The time to "cancel" is before you commit to buy the item (bid, make offer, accept offer, or Buy It Now, or Add to Cart and request a total).
There was an announcement of new procedures for cancellations a few days ago, but from what I read in the announcement I didn't see anything that would change the above.
