All imports regardless of being new or secondhand require you to pay VAT & Duty at the appropriate rate on first entry into the EU. Since VAT etc hasn't yet been paid on them (nice tax that VAT...)
http://www.hmce.gov.uk/channelsPorta...ument#P55_5483 £18 is the limit below which VAT is overlooked. (£36 for a gift)
£7 is the limit below which Duty is overlooked.
C&E aren't daft, and will ignore the label if the contents are obviously of higher value.
Whether these are levied or not is a different matter, but that's what your supposed to pay.
Even if you buy secondhand goods from a UK company that's VAT registered you should pay VAT on that as well (but may reduce it under the Margin scheme, where you would only pay VAT on the profit) so the Govt can essentially levy the VAT twice.
http://www.hmce.gov.uk/channelsPorta...HMCE_CL_001454