I've tried that angle. Doesn't work. Response precisely as you stated
"but you never do ...". Usually followed by reciting a list of the stuff
that I "could always sell later on ebay ..." and is still on the
premises.

Ah but that means that you must have succeeded with this strategy in
the past. My SWMBO does have a degree in rocket science from MIT so I
don't even get to try it the first time.

Have you tried the 'it turned out to be much more useful than I
expected' gambit?