More and more over the last couple of years I have noticed a massive influx of "925 stamped" sterling silver jewellery which is actually silver plated. Not all of it, by any means, as eBay is a fantastic place to pick up solid sterling silver items, one off artisan jewellery, vintage pieces and great bargains, but sadly there are a lot of mainly uninformed importers of jewellery, who are buying what they themselves believe to be sterling silver when it is in fact plated. What this means in effect is that the gorgeous bargain bangle you've just bought for £4.99 is actually worth about £1. You won't notice this straight away but a few weeks down the line after you've long ago left feedback saying how wonderful the bangle is, you'll notice it turning a bit coppery. Polishing it will just remove more of the silver, and you're left with something which a charity shop wouldn't sell for more than 25p.
Silver is an expensive metal and trade prices have doubled and then doubled again over recent years (I'm a silversmith, and am mortified by how much it now costs to make a small silver ring!), so it really is a case of getting what you pay for when it comes to sterling silver. A few years ago it was very rare for me to buy plated items on eBay that were claiming to be sterling silver. Now, sadly, around half of the items I buy are 925 stamped plated items, where the seller has listed them as solid 925 silver, or occasionally "925 stamped".
Please note that the 925 stamp is NOT a hallmark (which consists of a maker's mark, an assay office mark and then a fineness mark in the UK - this applies to modern pieces). Anyone can purchase a 925 stamp for around £5 and apply the 925 mark to whatever they wish. Look at my sterling silver tin of tomatoes, for example. It's 925 stamped so it must be silver!
I regularly buy bits of silver on eBay for finishing off projects of my own or for recycling into something new, but am very careful to file down an inconspicuous area of the item to make sure that it is in fact silver. You can use a fine needle file for this, or even an emery board will suffice. If you've just paid £1.50 for a sterling silver chain it really is worth your while to take a file to the clasp and double-check for yourself so that you can leave honest feedback on the item and prevent anyone else making the same mistake. What you'll notice very quickly upon filing the silver is that coppery gold coloured metal starts to show through. This is the cheap base metal that the piece is actually made of. It's a little hard to make out in the picture below but hopefully you'll be able to just about see the gold colour on the clasp of this "925 sterling silver snake chain" that I recently purchased on eBay. The seller was good enough to give me a full refund claiming that it must be a problem with their supplier (if I had a tenner for every time I have heard this one I'd be a very rich woman by now), but neglected to amend her other listings to include the fact that the chains are plated. What a shame, as at £1.99 each they would still sell very well and no one would have to suffer the embarrassment of giving one of these to someone as a gift and later discovering that the giftee is allergic to it.
Do note, however, that just because an item has only a 925 stamp without a hallmark does not mean that it is definitely plated. I myself only use the 925 stamp as hallmarking via the assay office is very expensive and would mean I would have to put my prices up considerably. But then, I'm one of those odd people who would feel horribly guilty doing that. (Perhaps that's why I'm so poor. Hmm). The only way to be sure is via electronic testing (expensive), acid testing (don't try this at home), or my filing method (which you have to do anyway with acid testing). If all of the above makes you never want to buy silver on eBay again please don't despair - there are also a lot of honest sellers on eBay who sell very fine 925 stamped silver jewellery which really is 925 silver at excellent prices, but beware that very fine silver jewellery isn't cheap. The current trade price for sterling silver (as of January 2012) is £795 per kilogram, meaning that a slim silver bangle costs around £15 in materials alone, without labour. Anyone selling brand new buy-it-now GENUINE silver bangles for £4.99 would very quickly go out of business. A brand new sterling silver bangle simply does not cost £4.99.
My general rule for buying silver is that if it's brand new, buy-it-now, and the price seems too good to be true, then it probably is just too good to be true.