There's a bit of  a big difference between - 'Over 30 items a year is a business seller' and a private seller flogging off about 300 items in a year from a huge mixed list of *used* items....

I'm talking about (amongst 1001 other items)--

A collection of used optical instruments. (late cousin was an optician)

Loads of 45 year old analogue electronic bits (late cousin was into audio as a hobby)

A load of old books about battleships and memorabilia of such ( F.i.L was in the navy) and the contents of his over filled toolshed and garage.

A collection of used (NOT-designer) shoes (M.i.L was the Imelda Marcos of the W. Mids)

Late S.i.L.'s collection of ex-library reference books (S.i.L was a librarian)

And 3 houses' worth of assorted kitchen, living room and household stuff , all used, some definitely in good nick and useable to somebody else....

As far as 'inherintance tax' and 'lawyer of possessions' goes.....?

S.i.L and cousin didn't even possess enough to go to probate. 

HMRC will not want to know.