Your idea needs to have a meeting with Mr Reality.
a) The business world is competitive, you are competing with the likes of Amazon who have a no-nonsense returns policy. If you become known as a business with a troublesome returns process, then customers will simply vote with their feet.
b) People returning stuff after short-use is, like it or not, a fact of life in today's world. Ask anyone who sells fashion goods ... girls buy stuff for the Friday night party and return it on Monday. It happens and it's basically a cost of doing business. If the stock is any good, it will get sold anyway, just as nearly-new instead of new.
c) In an increasing number of jurisdictions, the law falls on the side of the buyer. For example in the EU, your "exchange only" policy would be an absolute non-starter (with very few exceptions, e.g. food items, healthcare items etc).