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While there isn't a law requiring ecommerce stores to have a Return Policy, you may need one if you wish to enforce your terms and parameters of returns and refunds.
For example, in the state of New York, if a retailer does not provide a Return & Refund Policy, the retailer will be required to accept returns and give refunds for all merchandise returned within 30 days of purchase. If you wish to limit this in any way, you will need to create and post a Return Policy with your own custom terms.
Another reason for having a Return Policy even if not required by law is that most consumers prefer to shop at retailers with a clearly-posted Return Policy. This is especially important when shopping online, as more issues are likely when purchasing something without being able to see it in person first.
If you don't have a Return Policy, you may be at the mercy of state laws regulating return and refund timeframes. You may also lose potential customers who are concerned about not being able to return a product if they need or want to.