Although it's irritating that people don't reply immediately it has only been a couple of days and those haven't been working days. It's also been the hottest weekend this year so the seller may not be at their computer 24/7. Also Ebay messaging is not always the most reliable means of communcation.
If you don't get an email reply by possibly tomorrow request the seller's contact details and give them a call. If you still can't get hold of them then come back to AC and ask for further advice.
Hoping that won't be necessary and that your seller contacts you today, the first working day since you purchased the item.