Feature request (if not already): Buyer upgrade options on purchase
I don't know if this is possible, but if not then perhaps it's a feature request. We want to sell optional "add on" items, but only when a buyer buys the main item. Variations don't seem to fit this scenario, and secondary listings aren't a good fit either.
We have many craft items such as needlepoint canvases. If buyers don't already have yarns, we can create a second eBay listing for them to purchase the yarn as well, and sometimes we have 4 or more options for those yarns for a project. Yes we can use variations in a listing, such as one variation begin only the canvas and the other variations being the canvas with yarn, but it seems better if there was a feature for buyers to "add" any of the yarn options rather than to use variations. The main reason is because if we only have 1 or 2 canvases but list 4 variations, then we run the risk people buying too many variations before we can update the quantity available, and then we get dinged with out-of-stock order defects.
For now it's still better to create a second listing just for the yarn so we won't risk an out-of-stock defect, but we don't want to do this early because potentially someone can purchase yarn without the canvas, which doesn't help whoever is buying canvases if the yarn listings are gone and we only had enough for 1 project. Once, shortly after we listed yarn for a specific buyer, another person stumbled upon the listing, so it happens. But that second buyer made an inquiry, and we created a third listing specific to his request so it ended well for everyone (and everything was through the eBay platform).