I don't think there are any official or explicitly stated T&Cs for buyers, aside from the instruction to add items to the cart in order for the promotional discounts to take effect (and eBay don't even make it all that clear that it won't apply if you use the buy it now option on items instead - the heading is now at the end of the listing decriptions rather than the top, and while it does say "Discount will be applied when you add promotional items from [seller] to your cart", it doesn't say the discount won't be applied if you purchase through BIN. It's implied, but on eBay implying is often not enough 
).
I just found it because you told me to look at the bottom of a listing:
https://pages.ebay.com.au/specialoffers/buyerterms.html
The link is easy to miss:
Ah... It's good they do have it, but I completely missed it and I was looking for it 
(I was actually looking for a 'learn more' type link - tunnel vision 
).
