In the US, you must rely on seller policies and manufacturer guarantees . In Europe, you additionally have certain buyer's rights which are very significant and virtually always preferred over the former.
Since this is in Europe, let us first make a distinction between buying offline at a physical store, and remotely (e.g. online or via telephone).
In the offline case, you have to rely on store policy. If they offer returns, you can try and ask them nicely. Since stores compete with online merchants, it's likely they'll go the extra mile and let you return it or offer the price difference in cash or store credit to make you a happy and loyal offline customer.
On the other hand, if you bought it remotely, there are pretty customer-friendly regulations in place. You're allowed to return such an item bought for any reason within 14 days, paying only the return shipping costs (not for the initial shipping!). That is, minus a usage fee, if (and only if) the item lost value greater than its packaging cost, e.g. if you scratched it, and does not always apply to digital or digitalizable goods, and never to organic or similar goods. Of course, sellers will be glad to refund price differences to avoid returns. That customer-friendly law is also a reason why prices here are generally a bit higher :)
If the item were damaged or faulty at the time of purchase (not later!), they must accept its return (after repairing it up to two times, if they chose so) under general warranty in the EU, regardless if you bought it on- or offline. This topic is a bit complex in itself, with a 0.5 and a 2 year time period of importance, but I won't go into it as it is not applicable to your case.