In simple terms, it is a business operation when it becomes a
profit-making enterprise.
It is a grey area, but there is a difference between selling
occasional personal items on eBay and selling for profit.
I would imagine the sort of considerations HM Revenue & Customs would
take into account are the size of your turnover, the extent to which
you are both buying and selling, and whether you are clearly
specialising in one particular commodity as opposed of disposing of
unwanted presents or clearing the loft.
http://www.ebay.co.uk/gds/When-does-eBay-selling-become-taxable-/10000000004494855/g.html
I don't believe that you selling your personal camera gear will be taxable, but as the link says, it is a grey area.
They also recommend to do this
It's far better than having to deal with an investigation a few years
down the line.
When it comes to completing your tax return, there is a section which
is headed "other income", and it is here where you will enter the net
earnings from the web business.
"Net" here means your additional income, less all expenses associated
with it.
If you are still worried
I would always encourage people to take a cautious approach and discuss their position with HMRC via its helpline on 08454 915 4515.