First off, no store in their right mind has an "unconditional" returns policy.
There are always conditions, they are just so common sense that we skip them in normal talks. So for example you can unwrap a TV, plug it in, be dissatisfied with the way the on screen menu works and return it. That will be accepted as normal. You cannot take a pair of boots for a hike and give them back muddy and scratched and expect to get your money back.
There are no legal repercussions to enter into a contract and fulfil it one way or the other. If returning an item in the condition agreed upon is one option and you take it, that is fine. There is no "moral clause" in the contract to stop doing it on purpose.
However, merchants don't have to trade with you. Once a contract is entered they must follow through, but if they don't like what you do, you have no legal right to another contract in the future.
As a real world example, fashion retailers have very high quotas of returns, just because fashion is so horrible unpredictable. If you have ever ordered three exact same jeans on Amazon and they differed 5 cm in length because the brand has multiple factories and some were manufactured in Bangladesh and some in Egypt (I guess one of those countries hasn't invented the measuring stick yet?) you know what I am talking about. It is perfectly normal to order 3 items (the size you think you need, one smaller one larger) and send two back. Retailers still make money on that. Not the best customer, but still one worth keeping. But all of them keep track and all of them will rather lose a customer, than keep one that is costing them money by returning too much. Or damaged goods. Or goods where you can clearly see they were used for more than just seeing if it fits.
The contract you have formed is valid and you can excercise any option therein, but they will not offer you new contracts with those conditions, if they think you showed bad faith which made them no money on their last contracts.