Know your shop. Only buy clothes from stores or labels that you know and like. Avoid those Facebook ads (and on other social media) as it is likely to be a scam.
We buy a lot of clothes from eBay but they are all high quality labels (not trendy designer labels that are probably fake) and rarely have a problem. If I want new I only buy from stores I trust such as Charles Tyrwhitt for me and Celtic & Co for her.
Know the law. Goods you buy online can be returned for any reason but you must tell the seller within 14 days. If you just don't like them, they don't fit or you've changed your mind it is up to the seller's policy as to whether or not the cost of the return postage is refunded. If they are not as described or faulty, they must refund postage both ways.
Always return goods by a tracked method so you have proof.