The law in Germanyis equivalent. In France as well (as far as I can make out - I do have a copy of the French
regulations).
BUT
a) "Only firearms which have been proof tested (and marked as such) can be sold." Like the MOT test on your car, the proof mark only proves that it passed the test THEN - it is no guarantee of the state of the rifle NOW.
"Any changes to any pressure parts (bolt, action, bolt head or barrel) require the weapon to be re-proofed." Taken literally, that would also mean mean recrowning, rebluing, or just plain removal of rust. So how literally do you take it?
b) The regulations here are stringent. The chamber, for instance, must be pinned or welded shut in such a manner that it is impossible to insert a cartridge. And the firing pin must be removed or shortened. And the bolt face must be ground off at 45 degrees (i.e. so that it is screamingly obvious) etc etc. So the rifle that was the start of this thread would have been viewed not as a proper de-act, but rather as having an illegal alteration to a pressure bearing part that made the rifle unsafe (as was demonstrated).
But no private seller is going to give a buyer any kind of written guarantee. And verbal statements will, in practice, be useless: "I never said that" - "The buyer must have misunderstood me" - and so on.
So we are back to
"CAVEAT EMPTOR"