If the OP is referring to the UK Consumer Protection (Distance Selling) Regulations 2000 then a lot of the comments here may be misleading as they appear to refer to US practice. The UK regulations are weighted heavily against the seller. Note this quote from the OFT instruction leaflet.
The DSRs allow consumers to examine goods they have ordered as they would in a shop. If that requires opening the packaging and trying out the goods then they have not breached their duty to take reasonable care of the goods. In these circumstances you cannot insist that consumers return the goods as new or in their original packaging. You may ask consumers to return goods with the original packaging, but you cannot insist on this.
Read the complete OFT instructions for yourself or consult a solicitor.