The barrier to listing huge amounts of merchandise is not the commission, but rather the amount of labor it takes to list and price everything and pack and ship it and deal with refunds and returns for items that turn out to have problems. And how a lot of items are only economical to sell locally, because when you add in shipping costs it approaches the value of buying something new.
Buying eBay doesn't provide any kind of easy way to help offload inventory at all. The way inventory is offloaded in bulk is in bulk pallets sold at auction, where people bid on them and then do all of the grunt work involved in photographing and listing and packing and shipping. Which is a significant proportion of sales on eBay today. GameStop can easily auction off pallets of their merchandise if they want, today. In fact, there's a good chance they already do.