That is not a valid reason to force a cancellation under eBay rules. You may contact the seller and beg him/her to cancel your bid/offer/order (you use all 3 terms in your title/post but they are 3 different things) before the auction ends/s/he accepts the offer/ships the item, but s/he is under no obligation to do so nor to acknowledge the request, and neither not responding nor responding that it's OK with him/her nor that s/he doesn't know how is among the valid reasons.
If it is not as described when you get it (or if you don't get it by the estimated delivery date you get when you pay), come back with details for help on filing under the eBay Money Back Guarantee (which overrides the seller's "No returns" terms).
