FWIW-- Feedback is polite, but makes little difference to a seller's account.
It is good public relations and helps other buyers decide about the trustworthiness of the seller.
Feedback cannot be left after 60 days from purchase.
The transaction would probably be deleted from eBay's database.
If you ever have a problem with an item that has been deleted, you can settle it using Paypal's Resolution Centre which is available for 180 days from payment.