The easiest and safest cancellation is one before the item is shipped. You are not out any money because eBay will refund your fees as well, and if the buyer -did- leave negative feedback, you could ask eBay to remove it.
Rule of thumb is that if you force a buyer to pay, they will make you pay... either by claiming Not As Described, Damaged, or even Did Not Receive. Then you get the hassles and wasted time of fighting it... or giving them all their money back (and they likely will keep the item).
It bites when someone wins an auction and then requests a cancellation. If you had under-bidders (folks who were outbid and did not win), eBay does allow you to extend an offer to them, called a Second Chance Offer, to purchase at their highest bid amount. You decide if that's an acceptable price, and if it is, you send the Second Chance Offer -- under-bidders cannot force you to sell to them.
Cancel the purchase. Block the buyer if he really annoys you. Then check for under-bidders and see if, one at a time, any of them are interested in purchasing.
PS: a Second Chance Offer is like a fixed price listing available only to that buyer and for the amount of their last bid. You do not pay fees for this listing - even if you end up sending out 20 of them (some sellers will do that when they have multiples - auction one, then Second Chance a few more). If the buyer accepts, it's just like a regular listing purchase, and everyone's happy.
-Bob.
