I've just seen a post like that and... interacting with multiple people about different items on the same market entry would be in top 10 worst experiences of my life. I really find it weird not to post them separately.
Seems to work just fine. Typically they post a photo of the whole collection, and a text list of the items and prices, which they update as things sell. They're usually wanting to sell more than one game to each buyer, so it's much easier to coordinate in a single listing.