You can ask to cancel a sale but the seller does not have to agree.
Sending the item back would void you of any buyer protection under eBay's 30 day MBG, Money Back Guarantee policy.
The seller has sent it back to you?
Unusual, but as you have the item does the seller have a Returns Accepted on that listing? Then, you can return it at your expense for the reason, change of mind, for a full refund.
If the seller has No Returns accepted, you cannot return it, and perhaps try and sell the item yourself? Was it expensive ?
