Its not just the buyers
Many sellers, especially business sellers on ebay, don't know their legal obligations and have blatantly illegal terms and conditions.
OFT and Trading standards aren't helping a lot here. They do try to inform buyers of their *additional* rights to return unwanted goods, but explain that the seller can (if they make the right statements) require return carriage to be paid.
What they fail to adequately state is that Sale of Goods Act rights apply too, and that a seller cannot make the buyer pay return postage of DEFECTIVE OR INCORRECT goods, even if they have said they will for the return of purely-unwanted goods. The buyer can instead notify rejection of the goods, and the seller is entitled to collect if they wish, but the seller has to refund whether or not they collect the goods.
Nor can they make the buyer responsible for loss or damage in transit, Sellers are 100% responsible for loss or damage of goods in transit to a consumer.
Ebay of course takes no notice of reports, but there's no legal obligation on ebay to ensure its sellers don't make unlawful statements in their listings. Its Trading Standards not ebay that needs to be enforcing UK law.
A good start would be for OFT to set up a web site where buyers could report web sites with unlawful restrictions such as the above. Prosecution shouldn't usually be necessary. Instead, enforcers should check the site contains the stated phrase, confirm that the phrase is unlawful and notify the site owner of the problem -- initially by email, with a follow-up by recorded delivery if the offending content hasn't been removed. A sample of persistent offenders would need to be prosecuted as a deterrent to others, but in the common case (that the seller doesn't understand the law) it would achieve its aim with just a few minutes of the enforcer's time.