Too right it's risky thinking in terms of investment. Anyone who thinks they'll make megabucks from buying jewellery from selly telly and selling it on later is deluding themselves. I'd say you're better off putting a bit of money into buying gold sovereigns/half sovereigns. If anyone watches the antiques programmes on TV they will see just how low jewellery sells for at auction, and I'm talking vintage or antique pieces, often 18ct gold or a higher carat, and gemstone set, often with diamonds, and in the case of rings items with decent thick gold shanks, not the slivers of tissue paper gold bands that Gemporia regularly use for rings. You can purchase a good carat weight of 18ct gold ring for £100-£120 at auction and antique bar brooches for about £40-£60. OK, we know if you're buying at auction you have auction fees to pay on top, but still cheaper than shopping telly gold tone items.
Unless you're fortunate enough to snap up a real second-hand bargain, perhaps a rare item and for a cheap price - or you inherit a valuable piece to sell - then you won't make serious money from re-selling, in fact you'll often be lucky to make any money, so the advice to only buy if you love the piece is good advice. Sadly, the viewers being hoodwinked by the 'this is legacy jewellery' spiel will be in for a very rude awakening - and we remember the experience of poor Mirabelle back in the day.
You've mentioned the 'final warnings' of the Doom Merchants and those of us who watched Gemporia years ago will remember the 'buy now, before they disappear and won't ever be seen again' on repeat. Tanzanite, Alexandrite and Opals are just three I remember hearing about over and over again. The crystal ball was obviously wrong! Perhaps Lynn's crystal healing stones interfered with its' accuracy?