Silver and Sterling Silver (925) are two different things . There's no such thing as 925 SS plated either although you can get a platinum layer over 925 SS which eventually wears off but the 925 SS still won't permanently tarnish . It can tarnish but should come up again when polished. ( toothpaste is the best)

Silver plated means just a silver colored product , there's no silver plating for jewellery.

AND just because it has a 925 stamp on it does NOT mean it is 925 SS. Those Beads for 'Pandora Style" bracelets etc stamped 925 are 99% fakes and yes they will tarnish.

Also skin releases body salts and chemicals that will react to Tibetan Silver , Silver and even 925 SS tarnishing the item . SS will come up as it should but it's almost impossible to polish up Tibetan Silver and European silver to new.

So it's highly possible OP that while you may have purchased items that are stamped that doesn't mean they are and that the buyer may be correct in saying they have tarnished.

I just received 3 bracelets with Pandora Style lamp-work beads that are stamped 925 , I know they aren't even just going by the price I paid , but also because I have seen such items being "Stamped" on the markets throughout Asia 🙂 These are my personal items but even if I resold them I wouldn't claim them to be 925 Beads even if so many sellers on ebay do.