How to use Afterpay as a Seller?

You need to Register for Afterpay first before you can offer it as an accepted payment method Lorraine. This involves Trademe depositing 1c in to your bank account and you confirming back to them that you have received that via a 4 digit code they place as a reference with the deposit.

Full details here... Pay Now & Afterpay account – Help - Trade Me

Also if you are talking really expensive, it can't be used for items over $2000.

For over $1000, there can be buyer restrictions there as well. If the buyer already has a good history with afterpay they can usually grab those. But a newbie buyer or someone with previous defaults may have restrictions on value. I have a $1500 item that people regularly want to purchase with afterpay. Some seem to be able to do it, some can't.

Can I use AFTERPAY for certain items and not all of them . I have some cheap items which I don't want to use Afterpay for. I am just waiting for my code to update AFTERPAY and then I will be ready to go. Thanks for your help.

Yes Lorraine, Afterpay is an auction by auction optional payment method. You put it on auctions one by one of your choosing. It is not necessary to add it to everything you are selling.