I don't. I'm upfront with the buyer and very flexible with negotiations. My bikes usually sell fast despite my remote location. I usually offer excellent 'customer service' such as: good advice on parts/repair, incentives for return customers, bulk discounts, and if it's convenient for me, cheap or free repair (depends on the circumstance).

Since most of what I sell, sells inexpensively enough that it could be flipped for a profit by the new owner, I don't do returns. Come to think of it... I've only ever had one customer ask for a return, and he was trying to scam me.