...I get what you're saying - but comparing your average seller on Ebay to a retailer is apples to oranges.

If you are honestly selling "thousands" of things on Ebay - then you're probably approaching things more like a retailer than you are some random dude who has an item now and then they are selling.

Retailers build the cost of refunding returns, restocking, shipping, etc. into their overall cost of doing business. They know that a certain percentage of items will be returned, they know what kind of losses they'll incur from shoplifters, they know the cost of labor and what they need to charge to still make a buck while covering all of those things.

Random dude selling some item on the side probably hasn't taken all of that into account. He's probably just ready to get rid of whatever it is that he's selling and assuming that whoever bids on the item actually wants to purchase it.

Random ebay seller who has a few items he sells now and then (not like you and your thousands) is more like someone who has an occasional garage/yard sale every few years. And it would be weird if a couple of days after buying an item, someone showed up at the house and said "Yeah - that bookshelf I bought at your garage sale last week? I decided I don't want it after all. So here it is! Can I get my money back now?"