M40, you really dont seem to understand.
You sold the product by distance selling.
Which means he is covered by distance selling regulations
Which, as the vendor, you MUST honour.
Any solicitor who passed UK civil law (as in, all of them) will wash their hands of this immediately.
If you need this translating into the pigeon english you deem acceptable to use with your clients then I'm sure someone can do that. Your excuse of "too busy to bother" on something as menial as grammar, spelling and punctuation doesn't exactly fill people with faith in your work.
"excuse me, why is this product not up to standard"
"ye cuz iz 2 bizy to bee bovvered innit, yor problum deel wiv it"
Finally, this argument does not hinge on the quality of the product, even if it was the mst perfect piece of automotive art, your customer would still have all right to return it, without any reason, provided they let you know within 7 working days, starting from the date of which they recieve the goods.
I've tried to think of loopholes on how this could be struck out, the only possible route would be that no contract is formed, no "meting of minds", however, there was offer, acceptance, consideration and giventhat you are unknown to each other, I very much doubt you can argue no intention of legal relations.