Seller cancelled trade after I won

Hello doodah3. No. If it's gone it's gone. About all you can do is leave appropriate feedback to say what happened. A seller is not allowed to have an item listed on Trade Me if is also for sale elsewhere. 

Kia Ora doodah3,

It is extremely unfair and against TradeMe policy for Listing on the site, anything Listed onsite is not to be listed or for Sale elsewhere. - Your Seller needs to cancel their shop sale and complete the contract they have formed with you. - Terms & conditions clause 5.2

Trade Me site terms and conditions – Help - Trade Me

You will want to report the incident to TradeMe enabling them to take appropriate action.

If the item is long gone there isn't much you can achieve without legal action in which case the Citizens advice bureau may be able to advise you further on steps to take otherwise you'll have to leave it with TradeMe and continue your search.

The Help files refer...

Trade Me doesn't have the legal power to enforce a trade.

If the seller pulls out of a trade:

• Use the Disputes Tribunal to resolve the dispute.
• The Tribunal is less formal and faster than court.
• Apply online, by mail, or in person.
• Provide details of the dispute and attempts to resolve it.
• A referee hears the case and issues an enforceable decision.
• This process helps enforce the trade agreement when Trade Me cannot.