SKUs (stock keeping units) are a kind of product identifier, but they are an internal tracking number for individual businesses and not a standard code. A SKU basically differentiates products within the same company and helps sellers to identify their own catalog of inventory in the internal systems that they use.
eBay provides a field called “Custom label (SKU)”, which can only be accessed through eBay’s professional-level selling tools, and third-party tools. Within this field you can add your own identifier to each listing, to help you ship orders and update your listings correctly.
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So yes, SKUs are relevant to eBay sellers, but only if you have your own internal product identification system and need to match that up to your eBay listings. SKUs are not relevant at all to helping eBay uniquely identify products, like MPNs, UPCs, EANs etc.
One area of confusion with SKUs is that a brand or manufacturer might use it as their term for MPN. This is because the MPN really is their SKU – their own internal product tracking code. That doesn’t mean you have to use it as your SKU, and it is correct to use it as the MPN on eBay.