If you buy and sell items online to earn income as one of your side-hustle to supplement your primary job to reach your financial goals, and the payments are handled by eBay, or PayPal, then websites like finalfeecalc.com (also available on Google Play) or ebayfeescalculator.com (also available on Apple's App Store) can help with calculating and evaluating final value fees and profit to see if it is worth it before buying and selling an item.
finalfeecalc.com is simple and offers calculators for eBay and PayPal
ebayfeescalculator.com is much more detailed and offers calculators for countries other than the US and more stores other than just eBay, such as Amazon, Etsy, Cash App, Square, Copart, Stripe, Depop, and Reverb.