There is nothing for your buyer to mark as received, eBay only go by tracking numbers, and yes, eBay's tracking is slow to catch up with Royal Mail's / courier's tracking.
You are a new seller , so your payout will be held a little longer.
This was eBay' Announcement in August how payouts are made now:
From 6 August, we’re also making it easier and faster for our more experienced sellers to receive their funds. This means you can get your funds within 24 hours of the buyer paying. To qualify for faster payment, all you’ll need to do is:
- Complete at least 10 sales with a total of £150 or more within the last 5 years
- Maintain 2 or fewer cases in the last 12 months
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