It really is not acceptable, you are allowed to cancel immediately having placed an order without reason, this is generally interpreted as within 30mins or an hour , ebay trust sellers to honour cancellations unfortunately some sellers try and overcome this by stating the item has been dispatched which is the only legal way of declining such a request - ebay allow 60 minutes for a cancellation request to be made without penalty to either buyer or seller.
Sellers who choose to deny the cancellation request made immediately after purchase are either naive or lack knowledge that the buyer can simply use the money back guarantee to return the purchase for a refund either as a change of mind (remorse) return where the buyer pays the return postage or as a not as described ( fault ) return where the seller pays the return postage.
If you purchased from a personal seller the buyer can only return as a fault return where the seller pays the return cost.
What you can do is return the purchase once received choosing the relevant reason appropriate.
