Marketplaces are great as they provide more fulfillment options to the shopper. Different vendors may have different business models and thus pricing can vary and shipping distance and options can vary as well. e.g. I might be able to find Product A from Vendor B who is closer to me and thus the shipping is faster and cheaper. But it is very frustrating when the same product exists more than once in the catalog, or when different formats of the same product exist separately and are not connected. e.g. If you're viewing the DVD version, the marketplace should make you aware that there is a Blu-Ray version available as well. Look at the two examples below from the Amazon Marketplace. The DVD and Blu-Ray SKUs exist separately in the catalog -- the Blu-Ray product page (Image 1) makes the shopper aware that there is a DVD version. The DVD product page (Image 2) does not make the shopper aware that there is a Blu-Ray version. I happened to find the DVD product page first and executed a separate search to find out whether the Blu-Ray version existed or not.
Frustrated Shopper Suggestion
In a marketplace, eliminate and combine duplicate products so that customers don't have to worry about whether they are aware of all of their options.
That's all for now. Gotta go shopping!