There is no, and never has been, any way for a buyer to mark as delivered, their only option is to leave feedback.
Under the new payment system feedback left only works for items sent under the value of £10, then it can release the payout sooner.
Otherwise it stands thus:
Tracking showing delivery will release your payout 2 days after that delivery date.
No tracking ..... your payout is held for 14 days before release.
Read about the new Buyer Protection rules here:
https://www.ebay.co.uk/help/buying/paying-items/buyer-protection-fee?id=5594
@robthechop