Why change to 'IN Trade"?

The advantage is that buyers will see you adhere to the laws of NZ and are obliged to "do the right thing" for your customers. Buyers also are covered by the FTA & CGA when buying from an In Trade seller which if you are not showing as In Trade, it can be harder to push the point. 

Trademe have guidance in the Help pages here..

In-trade disclosure – Help - Trade Me

• The Fair Trading Act (FTA) applies to what people in-trade say and do when selling goods and services.
• The Consumer Guarantees Act (CGA) provides guarantees for when you purchase from a person in-trade.

Sellers must make it clear if they are in-trade when selling items online, so that consumers know they are protected under the FTA and CGA. 

plus, the law is explained on the Commerce Commission page and that gives further guidance on being In Trade.

Commerce Commission - Buying on Trade Me (comcom.govt.nz)

It's not that there is a benefit or not. It's what you legally have to do if you are "in-trade." You can't opt in or out just because you feel like it. You either are or you aren't. Very basically if you are selling something with an intention to make a profit, you are in-trade and that should be stated. 

As far as benefits, there are a few differences. A couple I can think of. You get access to the gold seller rebate for free shipping items. Buyers perhaps have more confidence buying from you because they are covered by both the Consumer Guarantees Act, and the TM buyer protection program if they pay by ping. So a benefit in that sense is that you can increase sales by declaring your in-trade status.

What  does in trade mean

HI Brian. If you click the link that Gary provided in his reply all will be revealed.

https://help.trademe.co.nz/hc/en-us/articles/360011642731-In-trade-disclosure