You said that the seller asked you to cancel the case as you were only returning the mattress. Did you do as he/she asked and cancel the case? If the answer to this question is "Yes" then eBay will have concluded that the matter had been resolved between you and the seller, and will not listen to any further cases in relation to the same transaction. If, however, you did not agree to close the case and kept it open then if the case is now showing as closed, despite no solution to the matter having been found, the first thing that comes to mind is that you may have left it too long and allowed the case to time out, in which case eBay would have closed it automatically.
In the event that eBay had decided the matter in your favour then you would have received a refund of the money that you paid to the seller and the case would have been closed at that point; however, you didn't specify as to whether or not you had received a refund. If you did get a refund but just wish to raise concerns about the seller you could try to phone eBay to raise your concerns, although it is better to do this during the morning when you are more likely to get through to somebody in Dublin who can speak English clearly, as opposed to phoning at another time and getting through to an eBay representative based outside of the British Isles, in which case you may end up speaking to an eBay representative who only speaks English as a second language.
In the event that something has gone wrong and eBay will no longer listen to anything you wish to say in relation to the transaction, then if you paid for the item using PayPal try opening a PayPal case against the seller instead. Unlike eBay, who only give you thirty days to open a case, PayPal allow buyers a period of one hundred and eighty days from the date of purchase to open a case against the seller. Therefore, this may well be your best option with regards to seeking a refund if eBay are not willing to listen to your case.
If you did not pay for the item via PayPal and paid via credit or debit card then if you have not yet received a refund try doing a chargeback and you should stand a chance of getting your money refunded.
Finally, if you have not already done so, leave some feedback for the seller to reflect how you feel the transaction went, making sure that you just leave fair and factual feedback that doesn't include anything that the seller could use as a justifiable reason to get the feedback removed, such as mentioning opening an eBay case, using derogatory language, etc. If the seller does then attempt to get the feedback removed but eBay cannot see anything that breaches their guidelines as to what can and cannot be said when you leave feedback then the comment is more likely to stick.
