Free returns is the phrase retailers use most loosely. On this list it means the retailer states in its own published policy that sending an item back does not cost you money. What it does not mean is that every return, from every customer, on every item, is free.
Read the condition, not the headline
Three patterns account for most of what you will find here. Some retailers make returns free without qualification. Some make them free for members, or above a spending threshold, or for the first return on an order. Some make them free only when they were at fault, meaning the item arrived damaged, faulty or not as described.
Those are very different promises, and the difference only shows up in the wording. Open a store page and read the quoted sentence before you rely on it.
Free to send back is not free overall
Watch for costs that sit outside return shipping. Original outbound shipping is often not refunded. Some categories carry a restocking fee that is deducted from the refund even when postage is free. Oversized items are frequently carved out of an otherwise free policy.
What to check before you buy
If free returns is the reason you are choosing one retailer over another, confirm three things: whether the item you want is in an excluded category, whether the free label depends on a membership you do not have, and how long the window is. A free return you cannot make in time is not worth anything.