This idea is fine for the initial sale of a piece but what
about reselling it down the line aways.
I think if you advise your initial buyer of the content of the piece of jewelry you have sold them, that's about all you can do. You can't be responsible for what happens 20 or 30 years down the road. They could have lost/damaged/given away/or whatever the piece of jewelry. How would you know and even if you knew, what could you possibly do about it. I would think your concern would be with the original sale and from there on it's the responsibility of the owner as to what they do with it.
Thank goodness we have a ‘proper’ hallmarking system in the UK. It
may be a bit more irksome to get a piece ready for sale but , in the
long run, it protects everyone down the line and makes life a lot
easier. At least when you buy something marked 14 carat, you know
for sure that every part of the piece including solder, fittings
etc., is at least that standard of metal.
Ian W. Wright