UK resident here!

Any business that has a turnover of £77,000 MUST register, some small businesses that have a lower turnover don't have to, but there are both advantages and disadvantages, ( it's a bit complicated but that is the gist)
If it was a ebay transaction, the seller might not have been registered for VAT, and in which case it would be illegal for him to charge the tax. My experience of dealing with the tax man and the VAT inspector , was that I did everything they told me, but I did it a little faster for the VAT man!