Licinius II, If The Seller Was Honest

Whether you agree with it or not, sellers can cancel a sale according to ebay. His reason is not the greatest, but neither you nor ebay can force him to send you a coin if he doesn't want to honor the transaction.

http://pages.ebay.com/help/sell/can...process.html

What "rules"? Guidelines and policies are set by respective companies and while I am not familiar with the policy of every company, I think that you will often find that bids and sales can be canceled- even though you might not have realized it.

for example here is the VCoins policy

"You agree that the order acknowledgment we send you by email or display for your viewing after you have placed an order is not a contract and does not signify our acceptance of your order, nor does it constitute confirmation of our offer to sell. VCoins and its member dealers reserve the right to accept, decline, or limit an order at any time after receipt for any reason. If an order can only be partially completed, you will be given the opportunity to cancel the order in whole or in part. A contract for sale is constituted only when an order is shipped."

here is the relevant bit from CNG under III.2 AUCTION TERMS (PUBLIC AUCTION)

"We reserve the right to reject any bid"