You must take care of the goods whilst they are in your possession, and either return them or make them available for collection by the seller. The seller may charge you the cost of collecting the goods or you may have to pay the return postage - unless the goods were faulty in the first place.
The suggestion is that the seller should pay for collection but it isn't very clear.
Reputable companies reimbursed any out of pocket expenses such as delivery charges if the goods supplied are faulty or fail to meet their description.
You cannot expect a company, however, to make a refund of delivery charges if you merely decide you don't want the goods after all.
Some of the big retail outlets, such as M and S, Tesco, Safeway etc have quite exceptional returns policies - all will not only refund your money if you decide you don't want something (providing it is undamaged), but Tesco and Safeway for instance, will give you double the original purchase price back if goods are faulty or groceries or perishables are not fit for consumption.
The basic rule of contract law is that if goods are faulty then the supplier has breached their contract with you and you are entitled to be returned to the original position you were in before the breach i.e. you should not be out of pocket, and therefore they should cover the P+P.
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