Just to set the record straight, ebay/paypal charges a little less than 15% on each sale.
Buyers pay for the shipping or the sellers builds the shipping cost into the sale price
Shipping is paid for the buyers or built into the sale price.
Of course the buyer winds up paying for it. I’m just saying that the bear’s share of profits are taken by the marketplace. They do that because they are providing the market place.
Just like a landlord at a brick and mortar store.