If the story policy is to allow you to be able return an item, within so many days, regardless if the item is used, then I think it's fine to do. If the stores policy is to only accept new items or defective items for a return, then I think an item used for a few days is a "used" item and would not fit into the returnable criteria. Lieing about whether or not it had been used would not be okay in my books (not that you were planning on lieing about it!).
Most of the stores that I've shopped at that allow used/tried (and not just defective) items to be returned, sell the item as "open box return" with a 5%-10% discount. What I like about this, is that future customers KNOW that the item has been opened and possibly used by a previous customer and can make the decision on whether they want to buy it. The store offers this to get you to buy items from them that you might otherwise decide not to purchase, and to hopefully have you exchange it for an item you DO like in their store.
I do agree that if the store doesn't do the "open box return" identification of an item and just puts it back on the floor, then I also would not feel good about returning an item I used. But, I also would no longer shop at stores that would put used items back on the sales floor without identifying them as returns.