eBay sellers who ask you to cancel transaction

If you receive a marble that isn't as described and are unhappy, do not hesitate to ask for a refund. Agree to submit a return. This way, you will get a digital label form to print for shipping. If a seller asks you to cancel a transaction as part of the return process, they are only ensuring you will not be able to leave feedback. This is how less than trustworthy sellers are able to keep a high positivity score.
Feedback is up to you.
Another way to help other buyers beware, is the star ratings, where you can rate the individual performance categories such as item description, communication, etc.