I put right in the listing if I do not take returns. I often sell in the Adult Only category where returns can be, well, just wrong. Sellers of swimwear, sex toys, easily duplicated DVD's, etc. often say, "Returns ONLY if item is defective." Or, "DVDs without the factory wrap are not returnable." Stuff like that. I mark that I accept returns when creating the listing, but put a very short time limit in telling me there is a defect. Then I repeat what I said in the listing. Some items are All Sales Final, and the buyer knows that. Some will try anything, though. I mean, depending on what you are selling, returns are just out of the question. That's it. It is my opinion that the buyer needs to READ THE LISTING, and fully understand what he is looking at. Sadly, many do not. And with the one-sided feedback system in place now, we sellers are screwed. I know you didn't ask for it, but my advice is, if you don't want to risk the chance of Negative Feedback, don't sell that item on Ebay. I haven't listed a new item for over a month because of that very thing.
When I buy an electronic part for my car at a brick-n-mortar, I know I can't return it after I install it and find that that was not the problem. I'm not going back into AutoZone to raise heck with the cashier, but Ebay bidders think it's OK to do that to the sellers.
It's no wonder so many have stopped selling there.