My father owned a pawn shop for many years and I am very familiar - actually, an EXPERT in gold and silver testing. I did it for him (up to less than a year ago) for about 20+ years.
*****About 95-98% of all people who claim " tested" for gold or silver, whether knowingly or unknowingly, are testing incorrectly.*****
A chemical test for gold OR silver only tests for the "presence" of a specific metal.... It does NOT test for plating, the most important consideration. If an item is silver plated with sterling silver as the plating, it will test positive as sterling silver.
Simply put, saying that an item tests positive for silver or gold is not the same as saying "solid". It means that where the chemical was applied, the "metal" for the test was present, and it turned positive.
For another example: You can also test an object to see if it there is a "presence" of 18k. If it is only plated, it WILL test positive if there is [18k]. Again, it is because [it] reacts with the 18k in the plating.
Testing acids are not "intelligent". They don't "know" they need to ignore the metal plating and "look" below the surface and test the metal below the surface metal. The solution only reacts to the existence of a substance, and if it is present (i.e. in the plating), even along with other metals such as brass, nickel, or aluminum below the plating, it will test positive and then you have a seller saying, "Not marked but tested positive for Sterling Silver", or "Not marked, but tested positive for 18k", etc.
The ONLY way, I REPEAT, the only way to genuinely test an object [not marked] is to take a file, remover a top layer of metal in any area of the item to remove any plating material, and carefully apply the acid to this area (without it touching the other unscratched area) and note the reaction. This is true of either the chemical tests or the electronic tests. A GOOD general rule of thumb for all professionals is: If it isn't marked .925, .900, SS, S.Silver, sterling,18k, 14k, etc, it is not. So bottom line, if it is marked, and in question... then test it. If it is not marked, then 99.999% of the time it is not even worth the trouble testing because it is not marked for a reason. Personally, I think most of unmarked sterling Ebay is not precious metal.
Granted, it is possible for anyone to get a small metal stamp, and stamp something on a item that is not gold or silver (Oddly, this is extremely rare. I have only seen this 5 -10 times in 20 years of a pawn business!), but it is even rarer that an artist or manufacturer would make a piece and then not stamp it.
This is especially true for Great Britain (they have VERY strict laws regarding the jewelry marking and hallmarks).
Believe it or not, Mexico is very good about marking their jewelry. Mexican and British jewelry are the finest in the world because of the strict and stringent marking guidelines by their governments. So, if it is being advertised as American, Britain or Mexican and the seller says "unmarked"... it is unmarked for a reason. It's unmarked because it isn't solid. The only exclusion to that I have seen on jewelry the bale is marked but the actual pendant is not. It is appropriate to do this, but ONLY if the bale is one solid piece and can not be removed from the item without cutting - then marking the bale and not the actual piece is appropriate. I have seen this on pieces like crucifixes.
Even the most basic, small, and rustic jewelry companies and jewelers (in ANY country) know to mark their jewelry. They know until they do, the item is in question until it is properly marked! If the "tested item" you just bought was tested, and it does NOT have a very noticeable scratched blemish somewhere (about the size of dime or just a little smaller) -or a "nick" in the side from using a file to obtain scrapings to (see below a surface to by-pass any plating) - it was not tested properly- or at all.
I have also seen, and I will try to type this without laughing, a lot of ALPACA "sterling", or "Sterling Alpaca" advertised. One seller just last week in the description listed the buckle as "Displays a stamp saying ALPACA but we are sure it is solid sterling even though not marked".
ALPACA is another term for nickel silver. It does NOT contain any silver content at all. Zero. Nada. None. Nothing. Period. I will say this again, NONE. Not even .001% Don't believe me? Google "ALPACA silver", or look [it] up on www.wikipedia.com and see what definition you get. I'll save you the time, it's plain old cheap nickel mixed with copper, aluminum, iron - or sometimes just cheap shiny pot-metal that photographs with a shine.
Ask your potential seller HOW (not IF) was the test done. You will be surprised by the answers you receive!
For some more GREAT information on sterling silver, search for a sellers guide written by eBay user sparklebizsilverjewelry. Great info in their guide too!