I've not read all the replies.
However. If parts are specially ordered, A retailer can refuse the right to refund IF they are correct for application and not faulty.
Most WILL charge a handling charge. I do. Mainly because it puts timewasters off and in some cases suppliers will adopt the same policy.
So to be fair the guy in question is at fault but its just unfortunate. As for the breaker. well.................
Goods made to the customers own specifications do not fall under the DSR's, but standard items ordered in specifically for the customer are covered by the DSR's and can be cancelled for a full refund.
Section 3.55:
"You are not allowed to make any further charges, such as a restocking charge or an administration charge."
If you are a business seller that takes orders over the phone/internet and mails out the goods, if the situation arises, you cannot refuse a refund. (Section 2.9: Compliance with the DSRs is a legal requirement.)
Obviously this ONLY applies to goods ordered by phone/internet etc. and mailed out, but a business seller CANNOT refuse a refund no matter why or in what condition the goods were returned, as mad as it sounds, the goods DO NOT even have to be returned prior to a refund being issued.
Even if the goods are returned damaged, the seller still cannot refuse a refund (see section 3.44).
All they can do, is take action against the consumer for breach of the statutory duty to take reasonable care of the goods, but a full refund MUST be given.
For mail order timewasters the DSR's are a dream come true.

DSR's do not apply to private sellers.