Will I get penalized if I have same items in multiple marketplaces, like Amazon, Ebay and my webstore?
Yes, websites can get penalized for duplicate content. However, when it comes to e-commerce products, search engines are smart enough to understand that tons of websites using the same default product manufacturers description is going to happen.
In that case, you likely won’t get a penalty. You may not get dinged by having the same description as everyone else, but you could stand out and get beneficial gains by taking the time to re-write your product descriptions to be unique.
If you’re not unique, why would search engines rank you uniquely above the rest?
To see just how diluted and competitive your products are, simply take a unique sentence within the product’s description, paste it within quotes into Google and see how many results there are that you’re competing against. Here is a random example of a Nike running shoe.
You can see there are tens of thousands of results with the exact same description.