How do I find the email address of a seller if I don't now have the email they sent at the time?

Follow the trade back to the original seller from your Ping notification. Then ask the seller a question on their other listings, and explain you've deleted their email address but would like to contact them.

Kia Ora oceanic,

If you are like me and have no Seller or Listing number details on your Ping payment email or your Ping Transactions from 6 months ago check your draft & sent emails if you corresponded with your Seller, asked questions and received answers or corresponded via payment instructions, Note: the business name you have provided is a store, the store is run by a different username therefore if you look the Store up through the site you can find their alternate username through their feedback page or the Bottom of one of their current Listings which may help you to find an email using that username - failing that your best option is to select the Bot, use the white back arrow at the Top Left of the window then select a New conversation on the blue bar at the bottom of the window to leave a message for the team through using the text suggestion box, enter the reference number from your Ping email or transactions so staff can find the Listing easily. It may take 2-3 days for someone to contact you as they are very busy currently however you will hear back via your email. TradeMe do support faulty goods and in this case may be able to help you gain contact with your Seller. - All the Best.

Kia Ora Callum,

The Ping emails are now very plain with little information - if it's a recent purchase I can select the product item which opens up the closed Listing for me where I have the Seller's email address however if it's an item passed 45 days which has left my won items, selecting the item from the Ping email brings up the closed Listing though without the header information supplying the email address. - Yes I have the Seller's name so can go to their other Listings though leaving a Q&A message regarding needing to be in contact the Seller can't supply their contact details - this is a store with multiple revolving purchases per day, I don't see them able to track back a 6 month email from a Buyer whose email address likely doesn't represent their TradeMe username? - Cheers.