Appears it may be site-wide as long as Afterpay is available as a payment method, but could likely excludes specific categories, particularly gift cards.
If you don't like the idea of paying in four installments, you could simply pay off the other installments straight after purchase.
Conditions: The offer entitles you to 10% off the purchase price (excluding postage costs) on Eligible items listed on www.ebay.com.au when you spend $100 or more in one transaction during the Offer Period and check out using Afterpay. A maximum discount of $150 per transaction applies. Multiple items may be purchased in 5 transactions (up to a maximum of 10 items per transaction). All monetary amounts specified in these terms and conditions are in Australian dollars (AUD).
Eligible Items means all items listed on ebay.com.au where these terms and conditions are found, and the coupon redemption code PAYIN4 is displayed in the item listing.
Which item did you try to add? Did it definitely show up as Afterpay available on the listing?
The following exclusions still apply:
Excluded categories
These are the eBay categories where Afterpay isn't available:
These categories won't show Afterpay available on the listing (same as stores that aren't eligible for afterpay either). You need to ensure that "or 4 payments of AU $XX.XX with Afterpay" shows on the listing.