There is is no “waiver”, no “as seen“that is valid in law. Whoever supplies any sort of work equipment must make sure that it safe to pass onto somebody else and they must describe any safety issues that exist with the equipment.

See http://www.hse.gov.uk/work-equipment-machinery/second-hand-products.htm

If you look at the link in the post by A Kurdziel, I think it is the 7th paragraph after thr heading "What must you do?", there are a couple of lines, then some bullet points, and a description of how you can sell used equipment legitimately.

It does state that a suitably worded waiver is acceptable.  How you word said waiver is anothe thing.  There are some suggestions on the HSE page.

Personally I wouldn't call it a waiver in the HSE document. Its clarifying the condition of the machine/equipment that you are selling/buying and what you still might need to do to make something safe/legally compliant.