Won an auction via Buy Now, but seller had already sold the item the day before on FB Marketplace?

Yes, lucky you hadn't paid. All you can do is contact Trademe to let them know, place a negative feedback to warn others. If the seller retaliates with feedback, just ask Trademe to remove it, they will if given all the info. Hope you can find what you wanted from a decent seller.

Happened to me too - give them negative feedback as Gary says.

Yes place negative feedback as a warning to others, I placed negatives 2 yesterday for Items I won and sellers pulled out,

both times I got Trades Mes involvement, as the excuses did not mount up at all?,

but most likely already sold off site, one had been paid for too,

I’ve had the same thing just happen and my payment went straight through. The seller has been apologetic and emailed to offer another product from their website, or refund. I’m disappointed as it’s clearly a breach of TradeMe’s Terms & Conditions.

4.4 Comply with our Policies
Auctions: When you list an Item for Auction on Trade Me, you should do so with the intention of selling this Item through the Site. Therefore you must not advertise your Item through another forum. If you remove a Listing for an Auction you'll incur a non-refundable withdrawal fee, regardless of the reason. You must sell to the highest bidder if the reserve price is met, or to someone who has bought using 'Buy Now'. You must only sell to a bidder who isn't the highest bidder after reasonable attempts have been made to contact the winning bidder without success. You agree to your email address being provided to the buyer when your Auction closes.

I’ll wait for my refund, post negative feedback and report through Community Watch

The seller has already sold the car to me on Facebook Market place but still has the 
running on Trademe. Can you force them to take the ad down please?

Kia Ora Ryan,

Have you paid in full for the car & when do you take possession of it?

It's in the Seller's best interest to keep the Ad up until they receive Full Payment - if the Sale were to fall through it will cost the Seller a second Listing Fee to Relist the Ad - Once it's taken down it can't be Relisted without creating a New Listing - it's set up this way to prevent Sellers from paying the 1 Listing Fee to sell multiple cars through hence avoiding New Listing Fees.

You are best to discuss your concern with your Seller for reassurance and you can make TM aware of your concern at the same time by contacting staff through the "Live Chat" facility - Cheers.