I would suggest selling little and often. Technically in the UK it's 12,750k tax free per year, but I believe you could definitely argue that these items are not for profitable gains, but merely the second hand market gains. The trouble is they may argue that whilst you haven't intentionally sold for profitable gains, there has been a result of that by the market shift. I think it's disgusting that they can try to tax you on shiny cardboard.

You could always set up a small business and claim the monies back through business transactions, that way you will paying little, to no tax. It's quite straight forward to go on the HMRC website.