There are a lot of refurbished iPods on eBay, where people take out the original drives and replace them with CF or micro SD cards, replace the backplates and screens, etc. I'm not sure how often this happens with GBAs but it's basically the same concept.
I would contact eBay directly to make sure, but you're probably okay to list them as long as your descriptions are extremely clear about what has been done to each item.