A viewpoint from the other side.....
I have never sold, but have bought many items over the years on eBay. I also ran a large, successful print and distribution business for over 20 years, shipping anywhere from dozens to thousands of packages a day globally, as well as importing printed materials, binders, and packaging products by OCC container from China.
If what I bought is not as described, and you want me to pay returned shipping, then you are going to be waiting a long time for it to ever come back, sorry. I will be glad to return the unwanted item immediately, at your expense. This is simply good business practice. Why should I incur extra expense if did not get what I thought I ordered? There is a lot of gray area here for sure though, and of course there is a difference in a $15 item and a $150 dollar item, or a $1500 dollar item.
Be honest, do everything in your power as a seller to rectify the situation at once. Not tomorrow, not next Wednesday . Offer a call tag, so the buyer does not have to pay return shipping, or even leave his house. FIX THE PROBLEM AND FIX IT RIGHT NOW. A bad transaction will happen from time to time. And I would much, much rather deal on eBay than Craigslist. The only time I buy on Craigslist is if the item is close enough for me to go see it in person, and then I make sure I take a friend with me. (although I am 6' 2" and weigh 245 pounds) Its just good sense to be prepared this day in age.
I have only ever had 1 bad transaction on eBay, where an item was broken in shipping. The seller did exactly the right thing. He had purchased insurance, and made all the arrangements to get the item back, all I had to do is wrap it back up and drop it at the Post Office. My money was immediately refunded in full, even the shipping, and I left him very positive feedback for his professional attitude and immediate fix. Will this always happen? Probably not, but its a small chance you take selling/buying on line.