Your seller SKU is also known as your MSKU or Merchant SKU. This is the unique SKU you assign to any product you are going to send into Amazon to be sold on their marketplace.
You can read a cool article about this here if you would like.
You have to keep your MSKUs under 40 characters at all times when you list products.
Best Practices for Healthy and Short MSKUs (Merchant SKUs)
1. Abbreviate your supplier names. For example, if you are doing retail arbitrage and sourcing a lot of books from Goodwill then shorten the name of your supplier to GW versus spelling out the entire name.
2. Don't add more than 4 SKU block configurations to your MSKU. Sure, you could add more, but how much data do you really need in your SKU? Shouldn't you just be feeding the Beast (Amazon)?
3. We give sellers the ability to create custom SKU tags to place in their custom SKUs but if you make super long ones then you will most likely go over the 40 character limit. For example, if you have employees listing products for you and his name is John Smith then maybe you create a custom SKU tag named "JS" instead.