Hello darrebaltoze0, For these old APIs like Searching APIs, Traditional Shopping/Buying APIs , Traditional Selling APIs, After Sale APIs, they are only using Auth & Auth token.
https://go.developer.ebay.com/api-documentation For new RESTful APIs like Sell APIs, Buy APIs (Beta), Commerce API (Beta), they are only using OAuth token.
https://go.developer.ebay.com/api-documentation Regarding your case, GetSellerList belongs to Trading API which means it is only using Auth & Auth token. You need to generate Auth & Auth toke to make the call in your code. Hope this can help you. And please let me know if you have any questions. Best Regards