Trademe sale has priority

IMHO, the first buyer if you believe they are legit, should be the one who gets to buy the item. BUT, if the second buyer (who did the "Buy now") have already paid for the item, then the "Buy now" buyer should get it since it's already been paid for.  Regardless of whether it's been paid for, you'll still risk getting a red face if you cancel the "Buy now" deal, so I'll still just sell to the "Buy now" buyer, and tell the first buyer "sorry".

I usually won't list an item on multiple platforms if I don't have multiple items in stock to handle the sales, I'll just list in the "other" selling platform if I'm confident it will be sold there, for obvious reasons...haha

Best lessons always learned the hard way huh? The Trade Me buyer would certainly be within their rights to make a complaint to Trade Me, and give you appropriate feedback if you don't complete the trade with them. If TM learns of your little error you'll get a slap from them as well. 

Here is the relevant rule that you signed up for when listing it on here:

Trade Me site terms and conditions

4.4 "..........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."